DREDGING  ENGINEERING 


PUBLISHERS     OF     BOOKS     FOR^ 

Coal  Age     *     Electric  Railway  Journal 

Electrical  World  v  Engineering  News-Record 

American  Machinist  v  Ingenieria  Internacional 

Engineering  8  Mining  Journal      *-     Po we  r 

Chemical  &   Metallurgical  Engineering 

Electrical  Merchandising 


DREDGING 
ENGINEERING 


BY 

F.  LESTER  SIMON,  B.  S.  IN  C.  E 

Assoc*  M.  AM.  Soc.  C.  E. 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    239   WEST  39TH  STREET 

LONDON:    6  &  8  BOUVERIE  ST.,  E.  C.  4 

1920 


COPYRIGHT,  1920,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


THE     MA1'I,K     FRBSS     Y  f  >  K  JC     I>  A 


PREFACE 

With  the  reader's  indulgence,  we  will  explain  just  what 
this  small  book  is  by  first  defining  what  it  is  not.  It  is 
not  a  compendium  of  statistics.  Neither  is  it  a  record  of 
the  actual  performance  of  specific  dredges  of  all  kinds  and 
under  all  conditions;  nor  yet  a  compilation  of  dredging  cost 
data.  Such  information  is  already  minutely  available 
in  the  Annual  Reports  of  the  Chief  of  Engineers,  U.  S. 
Army,  and  in  the  records  of  the  various  departments  of 
commonwealths  and  municipalities  controlling  dredging 
operations  in  their  respective  localities. 

It  has  been  the  author's  intention  first  to  describe  the  prin- 
cipal types  of  dredge  in  such  manner  as  to  impart  a  fun- 
damental working  knowledge  of  their  construction  and 
operation,  and  then  to  consider,  in  concise  form,  the  usual 
problems  confronting  the  engineer  in  the  conception  and 
accomplishment  of  dredging  projects. 

Because  of  the  fact  that  most  literature  upon  the  subject, 
having  been  presented  in  the  form  of  papers  and  articles 
in  the  technical  periodicals,  is  not  only  not  readily  available, 
but  incomplete,  in  that  each  as  a  rule  treats  only  of  one 
particular  phase  of  the  subject,  it  is  thought  that  a  need 
exists  for  a  comprehensive  treatise,  which  should  be  helpful 
alike  to  the  student  and  engineer.  To  fill  this  want  has  been 
the  author's  objective. 

The  importance  of  the  subject,  while  not  always  ap- 
parent to  the  layman,  is  obviously  paramount,  involving 
the  expenditure  of  many  millions  of  dollars  annually  for 
the  necessities  of  commercial  life. 

F.  L.  S. 

BALTIMOEE,  MD., 
April,  1920. 


Vll 


423635 


CONTENTS 

PAGE 

PREFACE .  ..- v  .,   „    .',..,    ,    .    ,    .  vii 

CHAPTER  I.     DEFINITION  AND  CLASSIFICATION 1 

CHAPTER  II.     GRAPPLE  DREDGES 4 

General  Description 4 

The  Bucket 6 

The  Common  Grab 6 

Bucket  Axioms '. •    •  H 

The  Sliding  Cross-head  Bucket 12 

Other  Types •  13 

Grab  Bucket  Details  and  Appurtenances 15 

Boom,  "A"  Frame  and  Back  Guys 16 

Spuds,  Spud-wells  and  Gallows-frame 20 

The  Machinery 22 

The  House 24 

The  Hull 24 

Operation 26 

CHAPTER  III.     DIPPER  DREDGES 29 

General  Description 29 

The  Bucket 29 

Boom,  Dipper-stick,  "A."  Frame  and  Back  Guys 31 

The  Spuds ..*...  37 

The  Machinery 39 

The  Hull ...;,.......  40 

Operation ........*.....  41 

Application  of  the  Type ^12 

High-powered  Dipper  Dredges 44 

CHAPTER  IV.     LADDER  DREDGES 46 

Historical 46 

General  Description 50 

Stationary  Type 52 

Sea-going  Hopper  Type 52 

CHAPTER  V.     Scows 54 

CHAPTER  VI.     HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 58 

Radial  Feeding  Dredge  with  Spud-anchorage  .........  58 

General  Description ..;... 58 

Cutter  Head  and  Ladder    .    .    ....    '.    .    ....    ....  61 

Feeding v   .........  64 

Boom,  "A"  Frame  and  Back  Legs  .    .  V  . 66 

The  Pipe  Line .  67 

The  Pump *  .    .  71 

ix 


X  CONTENTS 

PAGE 

Power 78 

Method  of  Design    ........  * 79 

The  Machinery 81 

Operation .'  * 83 

Booster  or  Relay  Pumps 83 

Forward-feeding  or  Mississippi  River  Type    ..........  86 

General  Description 86 

CHAPTER   Vll.     HYDRAULIC   DREDGES   OF   THE   SEA-GOING   HOPPER 

TYPE. ,  • 89 

Historical ';.  .    .    .  89 

General  Description   .     .  v /"".    .    .  91 

Advantages  of  the  Type 92 

Typical  Examples 93 


PART  II— DREDGING 

CHAPTER  VIII.     OBJECTS  AND  PHASES  OF  THE  SUBJECT 95 

CHAPTER  IX.  PRELIMINARY  ENGINEERING 97 

Exploration  of  Site 97 

Estimating  the  Quantities 100 

Choice  of  Plant  and  Method  of  Disposal  of  Dredgings  107 

Plans,  Specifications  and  Contracts 118 

Scheduling 121 

Estimating  the  Cost 123 

CHAPTER  X.  PRELIMINARY  CONSTRUCTION 125 

Dikes  for  River  Control 125 

Dikes  for  Impounding  Basins 130 

Dry  Dikes 130 

Wet  Dikes 132 

Spur  Piles 136 

Pressures  on  Wet  Dikes .  .  -.  ......  139 

Design 146 

Sluiceways.  . 150 

Pipe  Lines .-..>. 152 

CHAPTER  XI.  OPERATING  .......'.. 154 

Organization '  . 154 

Cut  and  Range  Layout 155 

Hydrography .  , 157 

Inspection >'-,, 158 

Basin  Regulation 164 

Progress  Keeping  . . . 167 

CHAPTER  XII.  REMOVAL  OF  SUB- AQUEOUS  ROCK 170 

Undermining  and  Blasting 170 

Drilling  and  Blasting  from  the  Surface 171 

Rock  Breaking  Machines 172 

Dredging  the  Broken  Rock 173 

INDEX.                                 175 


DREDGING  ENGINEERING 

PART  I 

DREDGES 


CHAPTER  I 
DEFINITION  AND  CLASSIFICATION 

A  dredge  is  a  floating  excavating  machine,  and  the  process 
of  removing  subaqueous  material  is  termed  dredging. 

There  are  many  kinds  of  dredges  and  several  classifica- 
tions are  possible,  differing  in  the  choice  of  a  basis  of 
distinction.  It  is  the  author's  intention  to  discuss  only 
the  more  important  types,  the  usual  equipment  of  to-day, 
disregarding  the  older  and  practically  obsolete  plant  such 
as  stirring  and  pneumatic  dredges.  With 'this  limitation, 
dredges  may  be  classified  broadly  under  two  general 
heads : 

I.  Bucket  Dredges 

II.  Hydraulic  Dredges 

Bucket  Dredges,  obviously,  are  machines  that  remove 
the  material  by  means  of  buckets,  which,  after  obtaining 
their  loads  by  biting  or  digging  into  the  bottom,  are  raised 
clear  of  the  water  to  dump  either  into  waiting  scows,  self- 
contained  hoppers,  or  upon  spoil  banks. 

Hydraulic  Dredges,  known  also  as  Suction  or  Pump 
dredges,  excavate  by  direct  centrifugal  pumping  through 
suction  pipe,  pump  and  discharge  pipe  into  hoppers  con- 
tained in  the  dredge  itself,  into  hopper  barges,  into  adjacent 
deep  water,  or  into  natural  or  prepared  reservoirs,  called 
impounding  basins,  from  which  the  great  volume  of  water, 

1 


2  DREDGING  ENGINEERING 

necessarily  pumped  along  with  the  dredged  material, 
runs  off,  leaving  a  deposit  of  solid  matter. 

The  bucket  class  may  be  sub-divided  into  three  types  : 

(a)  Grapple  Dredges 

(b)  Dipper  or  Scoop  Dredges 

(c)  Ladder  or  Elevator  Dredges 

Both  Grapple  and  Dipper  machines  have  a  single  bucket 
each.  In  the  former,  it  is  suspended  from  the  end  of  a 
swinging  boom  and  consists  of  two  or  more  shells  or  jaws, 
by  the  closing  and  opening  of  which  the  bucket  is  loaded 
and  discharged.  In  the  latter,  it  is  a  scoop  attached  to  a 
long  handle  and  "digs"  in  the  same  manner  as  the  familiar 
steam  shovel  on  land. 

Ladder  Dredges  consist  of  a  series  of  smaller  buckets 
travelling  in  endless  chain  succession  upon  an  inclined 
frame  called  a  ladder,  in  passing  under  the  lower  end  of 
which  they  receive  their  loads  by  scraping  along  and  into 
the  bottom,  discharging  into  a  chute  while  passing  over  the 
upper  tumbler.  Apparently  it  is  simply  an  application 
of  the  old  principle  of  the  bucket  elevator. 

Grapple  or  Grab  Dredges,  in  turn,  may  be  divided  into 
two  sub-types: 

(a)  Clam-shell  Dredges 

(b)  Orange-peel  Dredges 

A  distinction  only  by  virtue  of  the  style  of  bucket  carried. 
A  clam-shell  bucket  has  two  quadri-cylindrical  shells 
arranged  in  a  manner  sufficiently  analogous  to  those  of  the 
more  humble  clam  to  warrant  the  pilfered  title.  An  orange- 
peel  bucket  has  generally  four  shells,  forming  a  hemispheri- 
cal bowl  when  closed,  but  spreading  when  open  like  the 
quadrants  of  an  half  orange. 

Ladder  Dredges  are  susceptible  of  sub-division  into 
three  classes: 

(a)  Stationary  Dredges 

(6)  Self-propelled,  Barge-loading  Dredges 

(c)  Sea-going,  Hopper  Dredges 

The  first  is  the  usual  river  or  calm-water  type,  which  is 
fed  laterally  or  radially  by  means  of  anchorages  or  spuds 


DEFINITION  AND  CLASSIFICATION  3 

and  hauling  cables,  and  discharges  either  into  waiting 
barges,  or  into  deep  water  or  spoil  basins  more  remote 
from  the  dredge. 

Both  the  second  and  third  types  have  moulded  hulls  and 
sea-going  qualities,  but  the  second,  because  of  the  accom- 
panying barge,  is  confined  to  the  calmer  waters  of  ports 
and  estuary  channels,  while  the  third  is  a  sea-going  vessel, 
comprising  both  barge  and  dredge  in  one. 

Hydraulic  Dredges  are  of  two  general  types: 

(a)  The  River  Dredges 

(b)  Sea-going  Hopper  Dredges 
There  are  two  principal  River  Types : 

(a)  Radial  Feeding  with  Spud  Anchorage 

(b)  Forward  Feeding  or  Mississippi  River  Type 

Again,  Radial  Feeding  Dredges  may  be  either  of  the 
Swinging  Dredge  Type  or  the  Swinging  Ladder  Types. 
In  the  former,  the  dredge  itself  pivots  about  a  stern  spud, 
and  in  the  latter,  the  ladder  only  pivots  about  the  bow  of 
the  dredge,  which  is  held  stationary  by  four  spuds. 

The  subdivision  of  Hydraulic  Dredges  might  be  carried 
still  further  with  self-propulsion  as  the  distinguishing 
feature,  as  both  Radial  and  Forward  Feeding  Machines 
have  been  built  to  navigate  under  their  own  motive  power. 
They,  however,  are  the  exception  rather  than  the  rule  and 
would  result  in  unwarranted  complication  of  the  above. 

The  entire  classification  may  be  summarized  as  follows: 


Dredges 

1 

Bucket 

Hydraulic 

1 

1 

1 

1 

Grapple 

Dipper 

Ladder 

River 

,      1 

r 

1                     1 

Clam  Shell 

Orange  Peel 

Forward          Radial 
Feeding        Feeding 

1 

f 

1 

1                       1 

Stationary 

Self-Propelled 
Barge    Loading 

Sea-  Going                            Swinging       Swinging 
Hopper                               Dredge          Ladder 

CHAPTER  II 
GRAPPLE  DREDGES 

General  Description. — A  grapple  dredge  is,  in  principle,  a 
derrick  mounted  on  a  float  and  swinging  a  grab  bucket. 
Any  derrick  lighter  may  perform  the  operation  of  dredging 
by  simply  attaching  a  grab  bucket  to  its  fall.  This,  how- 
ever, is  merely  a  makeshift,  and  by  no  means  constitutes  a 
dredge.  The  operation  of  the  bucket  is  but  one  of  three 
principal  functions  of  the  grapple  necessary  to  the  complete 
business  of  " mud-digging."  They  are: 

1.  To  operate  the  bucket 

2.  To  control  the  position  and  local  movement  of  the 
dredge  itself 

3.  To  handle  the  scows 

The  first  requires  a  derrick,  comprising  boom,  "'A" 
frame  and  back  legs  and  the  requisite  hoisting  machinery, 
consisting  of  a  double  cylinder,  double  drum  engine  with 
boiler,  auxiliaries  and  appurtenances.  The  bucket  is  a  very 
important  machine  in  itself,  and  its  proper  design  is  most 
essential  to  the  efficiency  of  the  dredge.  It  will  be  discussed 
in  detail  subsequently.  It  is  hung  from  the  end  of  the  boom 
by  two  wires  or  chains  by  means  of  which  it  is  raised,  low- 
ered, opened  and  closed  and  by  which  also  the  boom  is 
swung. 

The  position  and  local  movement  of  the  dredge  is  con- 
trolled by  spuds,  by  anchors,  or  a  combination  of  both. 
Spuds  are  long  timbers  of  heavy  section  suspended  from 
tall  masts  or  so-called  gallows-frames  and  running  verti- 
cally through  ope'nings  in  or  pockets  attached  to  the  hull. 
They  are  raised  by  hoisting  engines  and,  when  released, 
penetrate  into  the  river  bottom  by  virtue  of  their  own 
weight,  thus  anchoring  the  hull  and  holding  it  in  position 
while  digging. 


GRAPPLE  DREDGES 


6  DREDGING  ENGINEERING 

The  third  function,  that  of  handling  the  scows,  is  accom- 
plished by  three  wires  attached  to  the  scows  and  lead  to 
capstans  or  to  hoisting  engines  on  the  deck  of  the  dredge. 
The  scow  is  thus  moored  to  the  starboard  quarter  of  the 
dredge  and,  as  each  pocket  is  loaded,  is  hauled  aft  until 
the  next  pocket  comes  under  the  bucket. 

Figure  1  is  a  first-class  representative  grapple,  the  Dredge 
"  Camden, "  a  7^  yard  clam-shell,  of  the  American  Dredging 
Company  of  Philadelphia,  shown  working  for  the  American 
International  Shipbuilding  Corporation  in  the  construction 
of  the  Hog  Island  Shipyard  on  the  Delaware  River. 

For  convenience,  in  a  more  detailed  exposition  of  the 
grapple  dredge,  each  component  part  will  be  discussed 
successively  as  follows: 

1.  The  Bucket 

2.  Boom,  "A"  frame  and  Back  Legs 

3.  Spuds,  Spud-wells  and  Gallows-frame 

4.  The  Machinery 

5.  The  House 

6.  The  Hull 

The  Bucket. — The  grapple  dredge  bucket,  or  the  grab 
bucket  as  it  is  called,  is  of  two  kinds,  the  clam-shell  and 
the  orange-peel,  as  briefly  defined  in  the  foregoing  classifi- 
cation. Both  have  the  same  principle  of  operation,  but 
differ  in  that  the  clam-shell  has  two  jaws  or  shells  while  the 
orange-peel  has  three  or  four.  There  are  two  principal 
types  of  clam-shell  bucket,  the  common  type  and  the  sliding 
cross-head  bucket,  each  of  which  may  be  distinguished 
further  as  a  hard,  medium  or  soft-digging  bucket  according 
to  the  class  of  material  for  which  it  is  designed.  Figure 
2  is  a  soft-digging  clam-shell  of  the  common  type. 

A  hard-digging  clam-shell  bucket  of  the  sliding  cross-head 
type  with  a  capacity  of  7^  cubic  yards,  is  shown  in  Fig.  1 
and  the  frontispiece. 

Figure  3  is  the  extra  heavy,  standard  orange-peel  bucket 
of  the  Hay  ward  Co. 

The  Common  Grab. — A  general  knowledge  of  the  con- 
struction and  operation  of  grapple  buckets  may  be  had  by 
an  analysis  of  the  common  grab. 


GRAPPLE  DREDGES  7 

The  bucket  has  five  main  constituent  parts  as  shown 
in  the  outline  drawing,  Figure  4. 

1.  The  two  shells  a 

2.  The  spool  and  shaft  b 

3.  The  four  arms  c 

4.  The  cross-head  d 

5.  The  two  closing  chains  e 


FIG.  2. — Soft-digging   clam-shell   bucket   of   the   common   type.      (Courtesy   of 
Vulcan  Iron  Works,  Inc.,  Jersey  City,  N.  «/".) 

The  shells  rotate  about  the  main  shaft,  to  which 
the  spool  is  keyed.  The  spool  consists  of  a  large  central 
sheave  to  the  perimeter  of  which  the  closing  wire  /  is  at- 
tached, and  two  cylinders  of  smaller  diameter,  (the  spools 
proper)  to  which  the  two  closing  chains  e  are  fastened. 


8 


DREDGING  ENGINEERING 


Whether  a  single  casting  or  independent  parts,  these  three 
wheels  are  keyed  to  the  main  shaft.  The  arms  are  pin 
connected  to  both  cross-head  and  shells.  The  upper  ends 
of  the  closing  chains  e  are  attached  to  the  under  side  of  the 


FIG.  3. — Extra  heavy  orange-peel  bucket.     (Courtesy  of  the  Hayward  Co.) 

cross-head  d.  The  closing  wire  /  is  confined  by  a  lead 
sheave  g  mounted  on  the  side  of  the  cross-head.  The 
opening  wire  h  is  bridled  to  the  cross-head. 

Obviously,  therefore,  if  the  bucket  is  suspended  from 
the  opening  wire  with  the  closing  wire  slack,  the  spool  and 
shells  fall  by  virtue  of  their  weight  until  the  spool  has 


GRAPPLE  DREDGES 


9 


rotated  to  the  point  at  which  the  closing  chains  are  vertical 
and  taut,  preventing  further  motion.  The  bucket  is 
hanging  open.  If,  now,  the  closing  wire  is  given  the  load, 
and  the  opening  wire  slacked,  the  central  sheave  is  caused 
to  rotate  so  that  the  spools  proper  are  rolled  up  on  the  clos- 
ing chains  toward  the  cross-head  until  the  bucket's  jaws 
come  together.  The  bucket  is  closed.  The  limit  of  ten- 
sion in  either  opening  or  closing  wire  is  the  weight  of  the 
loaded  bucket  in  air,  plus  an  allowance  for  impact  and 


Side  Elevation 
(Closed) 


Front  Elevation 
(Closed) 


Forces  Acting  on 
each  Shell 


Force  Polygon 
(One  Shell) 


FIG.  4. — The  common  grab  bucket. 

breaking  bottom  suction.  With  the  bucket  resting  open 
upon  the  bottom,  however,  the  closing  wire  tension  at  the 
beginning  of  the  "bite"  is  necessarily  less  than  the  weight 
of  the  submerged  bucket  empty  since  the  bucket  obviously 
would  be  raised  clear  of  the  mud  by  a  pull  greater  than  its 
weight.  As  the  bucket  closes,  the  weight  of  the  enclosed 
material  becomes  effective  and  the  wire  tension  increases. 
The  total  stress  in  the  two  closing  chains  is  equal  to  the 
product  of  the  closing  wire  tension  by  the  ratio  of  the  di- 
ameter of  the  central  sheave  to  that  of  the  chain  spools. 
The  total  compression  in  the  four  arms  is  equal  to  the  sum 
of  the  total  closing  chain  tension  and  the  weight  of  the 


10  DREDGING  ENGINEERING 

cross -head  divided  by  the  cosine  of  the  angle  6  between  the 
arms  and  chains. 

Isolating  one  shell  of  the  bucket,  (Figure  4)  the  forces 
acting  upon  it  when  closing  are  the  following: 

mn — the  thrust  of  the  two  arms. 

no — one-half  the  upward  pull  of  the  main  shaft. 

op — the  horizontal  pull  of  the  opposite  shell. 

pq — one-half  the  weight  of  shaft  and  spool. 

qr — the  weight  of  the  shell  itself. 

rm — the  resistance  offered  by  the  bottom  material  to  the 
cutting  lips  of  the  bucket. 

Beginning  at  m  and  going  clockwise,  the  force  polygon 
is  as  shown,  mnopqr.  The  force  no  represents  one  half  the 
sum  of  the  closing  wire  and  closing  chain  tensions.  The 
force  qr  acts  through  the  center  of  gravity  of  the  shell. 
For  any  given  position  of  the  bucket,  the  force  rm,  or  the 
cutting  resistance  at  the  lips,  will  be  maximum  when  the 
tension  in  the  closing  wire  is  greatest  (i.e.,  when  equal  to  the 
weight  of  the  submerged  bucket)  but  when  this  condition 
obtains,  the  bucket  is  on  the  verge  of  rising  so  that  the 
vertical  component  of  rm  is  zero,  or,  in  other  words,  the 
cutting  force  rm  at  the  lips  becomes  horizontal,  and  its 
value  may  be  determined  for  any  position  of  the  bucket 
by  taking  moments  about  the  main  shaft  as  a  center,  after 
computing  mn  as  above  and  approximating  the  weight  qr 
and  its  point  of  application.  The  form  of  the  shell,  there- 
fore, in  so  far  as  it  influences  the  magnitude  of  the  mo- 
ments mn}  qr  and  rm  about  the  shaft,  is  an  important  factor, 
i.e.,  the  shape  of  the  inscribed  triangle  formed  by  lines 
joining" the  main  shaft,  arm-pin  and  edge  of  lip  determines 
the  relation  between  the  lengths  of  the  lever  arms  of  the 
forces  acting. 

From  the  above  considerations,  the  following  conclusions 
may  be  drawn  in  reference  to  the  bucket's  ability  to  dig, 
or,  in  other  words,  its  closing  power  as  measured  by  the 
cutting  force  at  the  lips : 

1.  It  is  a  function  of  three  co-related  quantities:  the 
weight  of  the  bucket,  the  form  of  the  shell  and  the  ratio 
of  diameters  of  central  sheave  and  chain  spools. 


GRAPPLE  DREDGES  11 

2.  It  varies  inversely  with  the  depth  of  bucket  from  shaft 
to  lip. 

3.  It  varies  directly  with  the  width  and  weight  of  the 
bucket  and  with  the  diameter  ratio  of  sheave  to  spools. 

4.  Weight  in  or  near  the  curved  back  of  the  shells  is  no 
more  effective  than  that  concentrated  about  the  shaft. 

Bucket  Axioms. — The  bucket  designed  for  soft-digging 
need  not  have  inordinate  closing  power.  It  is  made, 
therefore,  as  light  as  consistent  with  strength  and  durability 
and,  in  shaping  the  shells,  the  consideration  of  maximum 
capacity  outweighs  that  of  the  adjustment  of  the  inscribed 
triangle  to  maximum  closing  moment.  The  ratio  of  the 
diameter  of  the  closing  wire  sheave  to  that  of  the  closing 
chain  spools  is  less  than  in  the  hard-digging  bucket. 

The  hard-digging  bucket  requires  great  weight  that  it 
may  obtain  a  full  load  in  refractory  material  and,  since 
the  maximum  combined  weight  of  bucket  and  contents  is 
fixed  by  the  power  of  the  main  engine,  bucket  capacity 
must  be  sacrificed  to  bucket  weight.  A  dredge  capable 
of  swinging  a  10  yard  soft-digging  bucket  will  carry  a  some- 
what smaller  hard-digging  bucket,  probably  about  7K  yards. 
The  relatively  large  sheave  to  spool  ratio,  essential  to  the 
hard-digging  bucket,  results  in  a  central  sheave  of  con- 
siderable diameter,  since  the  diameter  of  the  closing  chain 
spools  cannot  be  less  than  enough  to  reel  up  with  one  revo- 
lution a  sufficient  length  of  chain  to  close  the  bucket. 
The  designer  should  be  mindful,  however,  that  the  peri- 
meter of  the  sheaves  must  not  extend  below  the  horizontal 
plane  through  the  lips  of  the  open  bucket,  for  the-  reason 
that,  on  a  hard  level  bottom,  the  bucket  so  built  in- 
stead of  resting  on  its  two  lips  in  the  wide  open  condition , 
would  be  supported  by  one  lip  and  the  central  sheave  and 
would  have  to  close  partially  before  being  able  to  "bite." 
This  fault  is  negatived  in  the  soft-digging  bucket  by  the  non- 
resistance  of  the  bottom.  It  is  entirely  possible,  too, 
that  the  closing  power  be  excessive  in  proportion  to  the 
weight,  causing  the  bucket  to  close  too  quickly  before 
attaining  the  penetration  necessary  to  get  a  full  load. 


12  DREDGING  ENGINEERING 

The  depth  of  any  bucket  from  main  shaft  to  lips  must 
not  be  so  great  with  respect  to  the  width  that,  when  closed, 
the  main  shaft  is  so  far  above  the  horizontal  plane  through 
the  arm-pins  that  the  closing  moment  is  materially  reduced 
for  the  last  part  of  the  closing.  A  bucket  having  this  fault 
may  be  difficult  to  keep  closed  when  loaded  and  will  open 
very  quickly. 

The  curvature  of  the  shells  in  front  elevation  should  be 
sharper  than  a  full  quadrant  so  that  the  bucket  will  not 
fit  the  "bite"  so  neatly -as  to  create  a  mud  suction  resisting 
the  lifting  of  the  bucket. 

In  side  elevation,  the  lips  of  soft-digging  buckets  may 
be  straight  or  nearly  so  without  detriment,  but  in  the  hard- 
digging  type,  lips  of  considerable  curvature  are  more 
effective. 

All  buckets  should  be  so  designed  in  regard  to  shell  curva- 
ture and  maximum  spread  of  opening  that,  when  wide 
open,  all  parts  of  the  shells  may  lie  within  the  verticals 
through  the  lips.  Otherwise,  the  pressure  of  the  water 
upon  the  protruding  shell  surface  has  a  tendency  partly 
to  close  the  bucket. 

The  Sliding  Cross-Head  Bucket. — It  is  difficult  to  de- 
sign a  hard-digging  bucket  of  the  common  type  just  de- 
scribed that  will  combine  all  the  advantages  and  omit  all 
the  faults  mentioned.  Better  results  can  be  obtained  in 
tough  bottom  by  the  use  of  what  is  known  as  the  sliding 
cross-head  type,  Figure  1,  page  5.  It  consists,  in  principle, 
of  a  common  grab  bucket  supplemented  by  a  rectangular 
frame,  the  two  side  members  of  which  act  as  guides  for 
the  travel  of  the  cross-head  and  the  lower  member  of  which 
is  formed  by  the  main  shaft  of  the  bucket.  The  shells, 
instead  of  hinging  directly  on  the  main  shaft,  are  pin- 
connected  to  the  lower  corners  of  a  pair  of  triangular  links, 
which,  in  turn,  are  hung  from  the  main  shaft.  Thus 
the  hinge  centers  of  the  shells  are  below  the  main  shaft, 
with  a  consequent  decrease  in  the  length  of  the  lever  arm 
of  the  force  rm,  Figure  4,  resisting  the  closing  of  the  bucket. 
All  the  desirable  features  of  an  efficient  bucket  may  readily 


GRAPPLE  DREDGES  13 

be  embodied  without  the  presence  of  any  of  the  faults. 
In  addition  to  the  prime  advantage  of  great  closing  power, 
this  bucket  has  the  further  good  points  that  the  elevation 
of  the  spool  above  the  shell  hinges  allows  plenty  of  open 
spread,  keeps  the  spool  clear  of  the  contents  of  the  loaded 
bucket  and  raises  the  central  sheave  well  above  the  plane 
of  the  lips  when  open;  while  the  frame  adds  effective 
weight,  stiffens  the  entire  structure  and  provides  conven- 
ient fastening  for  the  bucket  poles.  On  the  whole  the  slid- 
ing cross-head  bucket  is  an  excellent,  durable  and  efficient 
tool. 

For  soft  digging,  however,  the  author  prefers  the  common 
grab  type,  as,  in  this  case,  greater  closing  power  is  not 
required  nor  are  bucket  poles,  and  the  frame,  by  adding 
unnecessary  weight,  reduces  the  capacity  of  the  bucket. 

Other  Types.— In  addition  to  the  above,  there  are  many 
types  of  grab  buckets,  some  differing  in  minor  detail  and 
several  in  closing  principle. 

Many  small  buckets,  i.e^  from  about  %  to  1%  yards 
capacity  are  rigged  to  close  by  a  three  and  four  sheave 
block  purchase  through  which  the  closing  wire  is  reaved. 
This  type  is  seldom  used  in  dredging  however. 

Buckets  of  larger  size  have  been  constructed  with  the 
arms  pinned  to  lugs  projecting  from  the  back  or  convex 
side  of  the  shells,  outside  the  bucket,  in  order  to  yield 
greater  closing  moment  by  increasing  the  lever  arm  length 
of  the  arm  thrust. 

The  Stockton  bucket  resembles,  in  principle,  a  huge 
pair  of  tongs,  to  the  ends  of  the  long  curved  handles  of 
which,  the  bridled  closing  wire  is  attached. 

The  Arnold  bucket  is  closed  by  compressed  air,  which 
drives  a  piston  in  a  cylinder  contained  in  the  bucket.  The 
object  is  to  correct  the  omnipresent  fault  of  the  common 
grab  consisting  of  the  loss  in  effective  closing  weight  due 
to  the  lifting  propensities  of  the  closing  wire. 

The  Williams  bucket,  page  14,  is  a  powerful,  capable 
tool,  unique  in  its  closing  power  arm. 

There  are  several  single-wire  buckets  on  the  market, 


14 


DREDGING  ENGINEERING 


FIG.  5.— The  Williams  bucket.     (Courtesy  of  G.  H.   Williams  Co.) 


GRAPPLE  DREDGES 


15 


but  they  are  seldom  used  in  dredging.  They  present  the 
advantage  of  ready  attachment  to  any  two-drum  hoist. 
Grab  Bucket  Details  and  Appurtenances. — Both  the 
cross-head  and  the  shells  may  be  either  monolithic  steel 
castings  or  built  up.  If  castings,  the  shells,  after  wearing 
away  at  the  cutting  edge,  may  be  fitted  with  attachable 
lips.  The  built  up  shell  is  made  of  steel  plates  varying  in 


FIG.  6. — Three  blade  orange-peel  bucket  in  operation. 

Co.) 


(Courtesy  of  the  Hayword 


thickness  from  about  %  to  %  inches  according  to  the  size 
of  the  bucket.  They  are  reinforced  at  the  corners  and  edges 
with  bent  plates  and  steel  castings  and  usually  at  one  or  two 
intermediate  points  in  the  width  of  the  curved  face  with 
plates  and  angles.  The  cutting  edge  or  lip  of  each  shell  is 
generally  strengthened  by  the  addition  of  a  steel  casting  or, 
preferably,  manganese  steel,  the  better  to  resist  the  severe 
abrasion.  The  arms,  in  section  are  variously  round,  square, 


16  DREDGING  ENGINEERING 

rectangular  and  "H"  shaped.  All  parts  of  a  bucket,  more 
particularly  the  pins,  shafts  and  pin  and  shaft  bearings 
should  be  generously  proportioned  and  provided  with 
renewable  bushings. 

The  capacity  of  a  bucket  may  be  increased  temporarily 
for  soft  digging,  i.e.,  in  material  that  has  sufficient  body  and 
cohesion  to  stand  up,  by  the  use  of  so-called  "  side-boards  " 
which  are  bent  plates  bolted  to  the  arms,  one  to  each  shell, 
and  having  the  effect  of  raising  the  backs  and  sides  of  the 
shells. 

The  tendency  of  the  bucket  to  rotation  must  be  pre- 
vented, as  otherwise  the  two  bucket  wires  would  become 
•crossed  and  fouled.  This  is  accomplished  in  one  of  two 
ways.  A  wire  called  a  "dorsey  wire"  is  attached  to  the 
side  of  one  of  the  bucket  shells  and  is  lead  by  sheaves  up 
through  the  boom,  about  midway  of  its  length,  to  a  small 
pendant  weight,  rising  and  falling  in  the  plane  of  the 
gallows  frame.  Or  a  pair  of  hardwood  poles  may  be 
fastened  to  or  set  in  the  uprights  of  the  bucket  frame, 
extending  up  through  rings  attached  to  the  boom  head. 
The  poles  have  the  additional  function  of  maintaining  the 
bucket  in  an  upright  position  on  the  bottom,  i.e.j  -preventing 
it  from  " falling  over"  when  landed  upon  sloping,  hard 
material.  For  this  reason,  they  are  considered  by  many 
" mud-diggers"  to  be  an  absolute  necessity  for  efficient 
dredging  in  hard  stuff. 

Teeth  are  rarely  used  on  large  buckets,  but  on  the 
smaller  types,  which  are  relatively  light  in  weight,  they  are 
often  helpful. 

Boom,  "A"  Frame  and  Back  Guys. — The  derrick  or 
crane  element  of  the  dredge,  by  which  the  bucket  is  handled, 
comprises  boom,  "A"  frame  and  back  legs  with  guys. 
The  "A"  Frame  is  usually  vertical,  or  nearly  so,  and  is 
set  some  distance  back  from  the  bow  of  the  hull  in  order 
better  to  trim  the  ship  and  to  permit  a  sufficient  length  of 
hull  forward  of  the  boom  heel  for  holding  the  scow  alongside 
when  the  first  pocket  is  being  loaded.  Back  guys  and 
usually  also  back  legs,  i.e.,  struts  paralleling  and  supporting 


GRAPPLE  DREDGES 


17 


the  tension  rods,  extend  from  the  peak  of  the  <CA"  frame 
down  to  the  after  deck.  There  are  generally  two  such. 
The  boom  heel  is  approximately  in  the  plane  of  the  "A" 
frame  and  its  upper  end  is  suspended  from  the  peak  of  the 
"  A"  frame  by  a  topping  fall  of  fixed  length  so  that  the  boom 


PLAN      Fig.  a 


Main  Sheaves' 
X 


Fig.  c  ^Vj* 

FIG.   7. — The  grapple 'dredge :   (a)  arrangement;  (6)  boom-end  forces;   (c)  boom 

end  force  polygon. 

has  a  constant  angle  of  inclination,  which,  for  loading  scows 
will  be  from  35  to  45  degrees  with  the  horizontal,  and  gen- 
erally less  than  35  degrees  for  long-boom,  banking  machines. 
The  two  bucket  wires  pass  over  main  and  guide  sheaves  at 
the  boom  end,  thence  between  the  so-called  "  Table 


18  DREDGING  ENGINEERING 

sheaves"  mounted  on  the  " table"  which  is  analagous  to 
the  horizontal  bar  of  the  letter  "A"  in  the  "A"  frame, 
and  from  there  to  the  drums  of  the  main  engine.  The 
two  sets  of  table  sheaves  are  spread  some  distance  apart 
athwartships  for  the  purpose  of  giving  the  bucket  wires 
enough  lead  to  the  boom  to  swing  it. 

The  weight  of  the  loaded  bucket  and  the  boom  and  the 
pull  of  the  engine  create  stresses  in  the  boom,  topping  fall, 
"A"  frame  and  back  guys,  which  are  readily  determined. 
The  manner  of  rigging  and  the  forces  acting  at  the  boom 
end  are  shown  in  Figure  7a  and  b,  and  the  stress  diagram  in 
Figure  7c.  For  the  analysis,  it  will  be. assumed  that  the 
closing  wire  lies  in  the  same  vertical  plane  with  the  boom 
and  that  it  takes  the  entire  load  of  bucket  and  contents. 
Since  this  wire  passes  over  a  sheave  in  the  boom  head,  the 
forces  ab  and  cd  representing  the  tension  in  it  must  be 
equal  to  each  other  and  to  the  weight  in  air  of  the  loaded 
bucket.  It  is  advisable  to  increase  all  stresses  by  an  impact 
allowance  of  at  least  50  per  cent.  The  boom  compression 
ad  and  the  topping  fall  tension  be  may  be  scaled  from  the 
diagram,  a  study  of  which  will  reveal  that  both  these 
stresses  increase  with  the  decrease  in  the  boom  angle  6 
and  also  with  the  decrease  in  the  angle  4>  between  boom 
and  closing  wire  to  engine. 

To  the  above  stresses  must  be  added  those  due  to  the 
weight  of  the  boom,  which  may  be  found  graphically  by 
drawing  a  second  force  polygon  of  the  three  forces,  end 
reaction  of  weight  of  boom,  boom  compression  and  topping 
fall  tension,  omitting  the  bucket  and  closing  wire. 

The  topping  fall  tension  creates  stresses  in  the  legs  of 
the  "A"  frame,  alternately  tension  and  compression  as  the 
boom  swings  and  maximum  when  the  boom  is  at  the  limit 
of  its  arc.  The  tension  in  a  back  leg  will  be  greatest  when 
the  boom  is  in  the  plane  of  that  leg.  A  graphical  analysis 
will  easily  yield  these  stresses. 

The  boom  is  designed  for  combined  compression  and 
bending  due  to  its  own  weight.  The  choice  of  section  is 
influenced  by  the  use  of  wire  cable  or  chain  for  the  bucket 


GRAPPLE  DREDGES  19 

operation.  Either  may  be  employed  with  equal  success, 
the  preference  being  largely  personal.  If  wire  is  used, 
the  sheaves  in  the  boom  and  on  the  table  must  be  of  large 
diameter  in  order  to  prevent  undue  bending  stresses  in  the 
cable.  The  end  of  the  boom,  therefore,  must  be  designed 
in  such  a  manner  as  to  provide  a  fork  between  the  prongs 
of  which,  the  two  large  sheaves  are  mounted  on  a  shaft. 
The  guide  sheaves  need  not  be  so  large.  If  chain  is  selected, 
all  sheaves  can  be  much  smaller  and  those  at  the  boom 
head  may  be  suspended  beneath  the  boom.  There  are 
many  types  of  boom.  In  the  smaller  machines,  it  may  con- 
sist simply  of  a  single  stick  of  timber  with  or  without 
truss  rods.  In  the  larger  dredges  it  may  be  built  up  of  two 
timbers  braced  together  side  by  side  with  sufficient  clear- 
ance between  to  contain  the  two  end  sheaves.  The  timbers 
are  often  reinforced  with  steel  plates  or  channels.  Or  it 
may  be  a  lattice  or  truss  boom  of  timber  or  of  steel.  A 
convenient  form  of  the  latter  comprises  four  angles  laced 
both  ways  and  with  deep  plates  at  the  ends  and  one  inter- 
mediate point  at  least.  Booms  of  this  type  require  trans- 
verse or  sway  frames  to  resist  racking  strains. 

The  "A"  frame  members,  if  built  of  wood,  must  be 
supplemented  with  steel  stay  rods  because  of  the  reversal 
of  stress.  Although  the  back  legs  are  subject  to  compres- 
sion only  in  rare  instances,  yet  it  is  advisable  to  combine  in 
them  both  strut  and  rod  the  better  to  hold  the  "A"  frame 
rigidly  and  truly  in  its  intended  plane  without  oscillation 
or  vibration. 

All  sheaves,  shafts,  boxes  and  the  derrick  fittings  should 
be  proportioned  generously  to  withstand  the  exceptionally 
severe  wear  and  tear  and  to  cut  repairs  and  renewals  to  a 
minimum.  The  connections  of  the  "A"  frame  and  the 
back  legs  to  the  hull  and  the  bearing  and  thrust  of  the 
boom  heel  casting  upon  the  hull  require  care  as  to  the  proper 
transmission  of  the  stresses  to  the  hull  and  will  influence  the 
hull  design  to  the  extent  of  the  provision  of  adequate 
strength  at  those  points. 


20 


DREDGING  ENGINEERING 


Spuds,  Spud  Wells  and  Gallows-Frame. — The  grapple 
dredge  is  held  in  position,  oriented  and  advanced  in  cut 
by  means  of  spuds  alone,  or  by  wires  alone,  or  by  a  combi- 
nation of  spuds  and  wires.  Many  machines  are  fully 
equipped  both  with  a  full  complement  of  spuds  and  spud 
handling  apparatus  and  with  all  the  necessary  machinery 
and  appliances  for  operation  by  wires  and  anchors.  This 
dual  arrangement  is  obviously  advantageous,  since,  while 
the  spud  method  is  more  convenient  through  the  saving  of 
lost  time  in  running  lines  and  anchors  and  through  the 


Anchor 


Anchor 


Anchor 


Anchor 
FIG.  8. — Grapple  dredge. 


Anchor 
Position  control  by  wires. 


non-interference  with  traffic  and  adjacent  structures,  yet 
there  are  times  when  the  wires  must  be  used,  as  in  deep 
water  or  in  swift  currents  or  in  the  event  of  the  breaking 
of  a  spud.  1 

For  position  control  by  wires,  five  lines  are  generally 
used,  two  quarter  lines  on  each  side  and  a  stern  wire.  They 
may  be  arranged  as  in  Fig.  8a,  or  as  in  Fig.  8b.  The  rigging 
of  8b  presents  advantages  over  the  former  in  that  the  two 
quarter  lines  forward  are  not  as  likely  to  foul  the  bucket 
in  digging  and  dumping  into  the  scow.  In  either  case,  the 
two  wires  on  the  scow  side  of  the  dredge,  usually  the  star- 
board, must  be  elevated  to  clear  the  light  scow.  This  is 
done  by  leading  them  from  the  drums  up  through  overhead 
sheaves,  one  attached  to  the  end  of  a  cross  arm  set  in  the 


GRAPPLE  DREDGES  21 

"A"  frame  or  gallows-frame  for  the  bow  wire  and  another 
mounted  on  a  post  or  column  erected  on  the  after  deck 
for  the  stern  quarter  line.  Some  machines  have  symmetri- 
cal wire  equipment  so  that  the  wires  may  be  elevated  on 
either  or  both  sides.  Others  have  but  two  spuds,  used  in 
connection  with  the  wires.  It  is  readily  seen  that  the 
high  wires  on  one  side  and  the  low  deck  wires  on  the  other 
exert  a  force  couple  tending  to  resist  the  listing  of  the  dredge 
in  the  direction  of  the  low  wires  when  the  boom  is  swung 
to  that  side. 

Another  expedient,  which  has  been  successfully  used, 
to  prevent  scow  and  other  interference  with  the  anchor 
wires  is  the  complete  submergence  of  the  wires  by  running 
them  through  sheaves  in  the  toes  of  the  spuds.  This 
arrangement  has  been  patented  by  the  Osgood  Dredge 
Company.  The  10  yard  clam-shell  dredge  "Finn  Mac 
Cool"  was  so  equipped. 

The  spuds  in  the  smaller  dredges  are  single  sticks  of 
round  or  square  timber.  In  the  larger  machines  they  are 
square  built-up  members  comprising  four  or  nine  pieces 
of  heavy  square  timber  bolted  together.  Some  spuds  are 
as  large  as  four  feet  square.  They  are  equipped  with 
pointed  metal  shoes  to  facilitate  penetration  into  the  bot- 
tom and  to  add  enough  weight  to  overcome  the  buoyancy 
of  the  timber.  All-steel  and  wrought  iron  spuds  are  some- 
times used,  both  round  and  square. 

The  spud  wells  or  openings  at  the  deck  through  which 
the  spuds  travel  may  be  either  holes  in  the  hull  or  housings 
attached  to  the  sides  and  stern  of  the  hull  and  called  "  out- 
side" wells.  The  spuds  must  have  sufficient  clearance  in 
the  wells  to  permit  perfect  freedom  of  operation. 

Three  spuds  are  required  for  control  entirely  independ- 
ent of  wires,  two  forward  at  the  sides  and  one  aft,  in  the 
center.  The  two  forward  spuds  are  suspended  from  a  tall, 
vertical  rectangular  frame  called  the  "  gallows-frame " 
approximately  in  the  plane  of  the  "A"  frame  and  often 
braced  to  it.  The  stern  spud  is  hung  from  a  frame  of  more 
humble  proportions  or  from  a  single  mast  maintained  plumb 


'2  "2  DREDGING  ENGINEERING 

by  guys.  The  gallows-frame  consists  simply  of  two  or 
four  columns  with  a  top  cross  cap  and  the  necessary  trans- 
verse struts  and  diagonal  braces  or  knees.  It  must  be 
borne  in  mind  that  the  load  upon  the  gallows-frame  may  be 
much  more  than  that  due  to  the  dead  weight  of  the  spud 
because  of  the  difficulty  frequently  encountered  in  pulling 
the  spud  out  of  the  tenacious  river  bottom,  i.  e.,  "  breaking 
the  suction. "  It  even  may  be  greater  than  that  developed 
by  the  maximum  pull  of  which  the  spud  hoisting  engine  is 
capable  due  to  the  fact  that  the  careening  of  the  dredge 
when  digging  is  resisted  by  the  reaction  of  the  spud  on  the 
high  side,  comprising  both  weight  of  spud  and  the  grip 
of  the  mud.  The  spuds  are  raised  by  the  spud  engine, 
having  a  wire  leading  through  a  sheave  at  the  gallows- 
frame  cap  thence  down  to  a  collar  encircling  the  spud  and 
free  thereof,  but  which  grips  the  spud  when  the  wire  is 
taut.  In  rarer  instances,  the  spud  wire  is  made  fast  to 
the  spud  well  housing  from  which  it  is  lead  down  through  a 
sheave  in  the  toe  of  the  spud  itself,  thence  up  through  the 
gallows  frame  sheave  and  finally  to  the  drum  of  the  engine. 
Eigged  so,  the  gallows  frame  need  not  be  as  high  as  the  full 
length  of  the  spud. 

It  is  interesting  to  note  here  that  in  the  case  of  a  dredge 
swinging  a  long  boom,  excessive  listing  is  sometimes  pre- 
vented by  the  use  of  boom  logs  attached  to  the  sides  of  the 
hull  by  chains  of  such  length  that  the  logs  are  afloat  when 
the  machine  is  on  an  even  keel,  but  are  raised  clear  of  the 
water  when  the  boom  swings  to  the  opposite  side. 

The  Machinery. — The  usual  machinery  of  the  clam  shell 
dredge  comprises  the  following  units : — 

1.  The  Bailer:    The  Scotch  Marine  is  a  general  favorite 
although  Locomotive,  Leg,  Vertical  and  others  are  used. 
It  is  usually  designed  to  carry  from  125  to  150  pounds 
pressure,  and  is  located  aft,  where,  with  the  appurtenant 
coal  bunkers  and  water  tanks  it  acts  as  a  counterweight  to 
the  active  loads  forward. 

2.  The  main  engine  for  operating  the  bucket  wires:  gen- 
erally a  two  cylinder  horizontal  type  driving  two  drums 


GRAPPLE  DREDGES  23 

through  pinions,  spur  gears  and  frictions.  The  drums  are 
in  line  athwartship  and  spread  to  give  a  direct  lead  to  the 
table  sheaves.  The  frictions  may  be  either  of  the  block  or 
disc  type,  the  latter  being  preferable  for  large  machines. 
The  pinions  are  as  a  rule  below  the  elevation  of  the  drum 
shaft.  In  the  older  dredges,  the  drums  were  thrown  into 
clutch  with  the  frictions  by  means  of  a  long  hand-operated 
lever  in  the  pilot  house.  Now,  however,  steam  or  com- 
pressed air  is  used,  except  in  small  machines.  The  fric- 
tions are  a  very  important  item  of  the  unit  and  should  be 
given  careful  consideration.  For  efficient  digging  their 
grip  must  be  positive  and  their  release  quick  and  complete. 

3.  The  Secondary  Engines:    The  operation. of  the  spud 
hoists,  the  anchor  wire  drums  and  the  capstans  or  drums 
(as  the  case  may  be)  for  scow  lines  may  be  accomplished 
through  two  engines  if  desired,  one  forward  and  one  aft. 
If  the  dredge  be  fully  equipped  with  a  complete  comple- 
ment of  spuds  and  anchor  wires  and  with  symmetrical 
scow-handling  equipment  for  right  and  left  hand  digging, 
a  total  of  14  drums  or  drums  and  capstans  are  required, 
3  for  the  spuds,  5  for  the  anchor  lines,  and  3  on  each  side 
for  scow  control.     The  secondary  engines  should  be  double 
cylinder. 

4.  Pumps:     A  minimum  of  two  pumps  is  essential,  one 
for  the  boiler  feed  and  the  other  for  pumping  from  the  bilge, 
water  tank  or  water  scow  and  for  fire  purposes.     If  a  sur- 
face condenser  be  used,  circulating  and  air  pumps  will  be 
required. 

5.  Condenser:    The  main  and  secondary  engines  may  be 
run  as*  condensing  engines  by  piping  the  exhaust  steam 
from  all  to  a  single  condenser. 

6.  Miscellaneous  equipment,  such  as  electric  light  plant, 
air  compressor  and  refrigeration  plant. 

The  runner's  control  in  the  pilot  house  comprises  simply 
main  engine  throttle  and  frictions.  The  secondary  engines 
are  under  local  control  upon  signal  from  the  pilot  house. 

An  idea  of  the  relative  engine  sizes  may  be  had  from 
the  following  data:  The  dredge  "ADMIRAL/'  shown  on 


24  DREDGING  ENGINEERING 

the  frontispiece,  has  a  hull  110  ft.  X  39  ft.  X  11  ft.  10  in., 
swings  a  7>2  yd.  bucket  and  has\  horizontal,  two-cylinder 
main  engine  20  in.  X  24  in.  The  dredge  " BALTIC," 
American  Dredging  Company,  has  a  hull  110  ft.  X  39  ft. 
X  12  ft.,  a  5J^  yd.  bucket  and  a  horizontal  two-cylinder 
main  engine  16  in.  X  24  in.  The  dredge  " COLUMBIA," 
of  the  same  Company,  has  a  hull  90  ft.  X  35  ft.  X  10  ft.,  a 
4  yd.  bucket  and  a  two-cylinder  horizontal  engine  14  in. 
X  20  in.  The  "PACIFIC"  (same  owners);  hull  78  ft.  X 

23  ft.  9  in.   X  7  ft.,  bucket  2>4  yd.,  main  engine,  two- 
cylinder,  horizontal  10  in.   X   15  in.     The  dredge  "FINN 
MAC  CQOL;"  hull  120  ft.  X  40  ft.  X  12  ft.  6  in.,  bucket 
10  yd.  (soft  digging)  main  engine,  two-cylinder,  18  in.  X 

24  in.     The  " ADMIRAL"  mentioned  above  will  swing  a  10 
yd.    soft-digging   bucket.     Dredges    of   this    size   usually 
have  secondary  engines  of  from  8  X  10  to  10  X  12  double 
cylinder.     The  engines  of  clam  shell  machines  are  seldom 
run  condensing. 

The  House/ — The  usual  grapple  boasts  a  two-story  frame 
or  steel  house,  the  first  floor  of  which  comprises  boiler  and 
engine  housing,  galley  and  mess  room;  and  the  second 
floor,  pilot  house  or  operators  room,  and  sleeping  quarters 
for  the  crew  and  inspector.  Deviations  in  detail  from  this 
arrangement  are  not  uncommon.  The  main  engine  and 
boilers  are  generally  depressed  below  the  main  deck. 

The  Hull/ — The  dredge  hull  must  be  of  sufficient  size 
to  contain,  with  comfortable  freeboard,  all  the  above  men- 
tioned superstructures  and  machinery  with  adequate  fuel 
and  water  storage.  The  beam  and  the  length  forward  and 
aft  of  the  "A"  frame  must  be  sufficient  to  provide  adequate 
stability  when  dredging,  i.  e.,  to  keep  the  ship  in  reasonable 
trim  in  resistance  to  the  listing  moments  of  the  swinging 
boom  and  bucket.  The  width  of  hull  cannot  be  so  great, 
on  the  other  hand,  as  to  necessitate  an  excessive  length  of 
boom  in  order  to  reach  beyond  and  clear  the  pocket  coam- 
ing of  the  light  scow.  The  freeboard  should  be  such  as  to 
assure  some  reserve  buoyancy  when  the  machine  is  at  the 
point  of  maximum  inclination  due  to  the  limiting  position 


GRAPPLE  DREDGES  25 

of  the  loaded  bucket  in  its  arc.  In  brief,  the  problem  is 
the  proper  co-ordination  of  hull  dimensions  with  the  loca- 
tion and  height  of  "A"  frame,  length  of  boom,  bucket 
capacity  and  disposition  of  machinery,  fuel  and  water  tanks. 
The  solution  is  simplified  by  the  commonly  rectangular 
shape  of  the  hull  both  in  plan  and  section,  with  rake  at  the 
stern  only  to  facilitate  towing.  Moulded  hulls  have  been 
constructed  for  grapples  but  are  quite  rare.  The  total 
depth  of  hull  is  the  sum  of  the  draught  of  the  empty  hull, 
the  displacement  depth  due  to  machinery,  superstructures, 
fuel  and  water,  and  the  desired  freeboard.  The  first 
quantity  is  first  approximated  by  roughly  estimating  the 
total  feet  board  measure  of  lumber  in  the  hull  (or  the  ton- 
nage, if  steel)  and  checked  subsequently  from  the  accurate 
bill  of  material  taken  from  the  detailed  design.  The 
ratio  of  hull  depth  to  the  width  is  as  a  rule  slightly  less 
than  1  to  3  and  that  of  beam  to  length  a  little  more  than 
1  to  3. 

The  principal  loads  acting  upon  a  dredge  hull  are  the 
normal  water  pressure  on  bottom  sides  and  ends;  the 
weight  of  the  machinery,  more  or  less  concentrated  under 
the  main  engine  and  boilers ;  the  pull  of  the  main  and  secon- 
dary engines;  the  weight  of  fuel,  water  tanks  and  super- 
structure; the  thrust  of  the  heel  of  the  boom  which  may  be 
in  any  vertical  plane  passing  radially  through  the  heel 
casting;  the  alternate  thrust  and  pull  of  the  A  frame;  the 
pull  of  the  back  guys;  the  bearing  of  the  gallows-frames; 
horizontal  force  couples  due  to  spuds  in  their  wells  or  to 
anchor  wires;  and  finally  wave  action,  causing  both  local 
impact  and  bending  moments  in  the  structure  as  a  whole. 
While  the  majority  of  the  above  loads  are  capable  of  reason- 
ably accurate  determination,  it  is  hazardous  to  design  upon 
a  purely  theoretical  basis.  The  efficiency  of  a  dredge 
depends  among  other  things  upon  the  ratio  of  its  working 
time  to  the  total.  The  more  the  time  lost  for  repairs  and 
renewals,  the  less  valuable  is  the  unit.  Therefore,  either 
temper  the  theory  with  the  knowledge  resulting  from  prac- 
tical experience  or  else  use  a  large  safety  factor,  to  the  end 


26  DRE])GING  ENGINEERING 

that  the  members  shall  be  proportioned  generously  to 
withstand   the   severe   duty   required   of   "  mud-diggers. " 

Most  hulls  are  built  of  wood.  They  are  virtually  heavily 
constructed  boxes  stiffened  with  bulkheads,  trusses  and 
knees  to  resist  distortion.  A  hull  that  has  become  convex 
upward  longitudinally  is  said  to  be  " hogged"  and  when 
concave  upward,  "dished."  Although  varying  widely  in 
detail,  the  usual  design  is,  in  principle,  as  follows:  The 
bottom  planking  is  laid  transversely  upon  the  under  side 
of  longitudinal  keelsons,  heavy  timbers  spaced  from  about 
2  ft.  6  in.  to  4  ft.  c.c.,  extending  the  full  length  of  the  ship 
and  spliced  with  long  scarf  joints  from  4  to  6  ft.  in  length. 
The  reactions  of  the  keelsons  are  taken  by  heavy  cross 
keelsons,  running  athwartship  at  greater  intervals  on  top 
of  the  keelsons  and  notched  down  and  over  them.  Upon 
the  cross  keelsons  in  turn  bear  the  longitudinal  bulkheads 
or  trusses  and  the  stanchions.  There  are  at  least  two 
trusses,  usually  of  the  Howe  type,  or  solid  bulkheads 
extending  the  full  length  of  the  hull  which,  with  the  keelsons 
and  side  planking  (acting  also  as  deep  longitudinal  girders) 
furnish  the  requisite  stiffness  fore  and  aft.  Upon  the 
trusses,  bulkheads  and  stanchions  are  placed  the  deck 
beams  carrying  the  deck  planking.  More  commonly  the 
deck  beams  are  transverse  members  and  the  decking  longi- 
tudinal. The  deck  is  usually  crowned  about  3  inches. 
The  side  planks,  called  "strakes,"  are  spiked  to  the  side 
stanchions,  the  thrust  of  which  is  transmitted  to  the  cross 
keelsons  and  the  deck  beams  by  fore  and  aft  ribbon  pieces 
sometimes  called  side  cleats.  Frequently  two  or  more  side 
strakes  are  thicker  than  the  others,  extending  beyond  the 
side  plane  and  acting  as  fenders.  The  inclined  members 
of  the  stern  are  called  rake  timbers.  All  the  exposed 
planking  is  dressed,  outgaged  and  caulked  with  pitch 
and  oakum.  Deck  spikes  are  covered  with  wood  plugs. 
Transverse  stiffness  is  provided  by  lateral  bracing  or  by 
hackmatack  knees. 

Operation. — The  dredge  is  towed  to  the  site  of  the  work 
and  placed  in  position  at  the  starting  point  of  the  project. 


GRAPPLE  DREDGES  27 

Her  spuds  are  dropped  or  lines  and  anchors  set  as  the  case 
may  be.  The  cut  to  be  dredged  is  indicated  by  range  tar- 
gets stationed  ahead  of  the  dredge.  A  light  scow  is  brought 
alongside  by  the  tug  acting  as  tender  and  moored  to  the 
machine  by  the  wires  of  the  scow  handling  machinery, 
which  are  generally  3  in  number  located  as  follows:  A 
breast  wire  leading  from  a  "U"  bolt  in  the  pocket  coaming 
of  the  scow  to  the  port  bow  of  the  dredge;  a  second  wire 
attached  to  the  port  stern  corner  of  the  scow  and  running 
forward  to  the  starboard  bow  of  the  machine;  and  a  third 
wire  extending  from  the  starboard  stern  of  the  scow  to 
the  starboard  stern  of  the  dredge.  The  arrangement  is 
shown  in  Figure  7,  page  17.  It  is  customary  to  interpose 
a  boom  log  between  dredge  and  scow. 

The  deck  hands  board  the  scow  and,  using  bars  as  levers, 
wind  up  the  door  chains  of  each  pocket  about  the  shaft 
until  the  doors  are  raised  to  the  closed  position  and  held 
so  by  ratchet  and  pawl.  The  end  pocket  nearest  the  dredge 
having  been  thus  closed,  digging  is  started. 

The  operator  in  the  pilot  house  releases  the  starboard 
friction  to  slack  the  closing  wire  and  the  bucket  opens 
hanging  by  the  port  wire.  He  then  slowly  lowers  the  open 
bucket  into  the  water  by  easing  up  the  port  friction  until 
the  bucket  rests  on  the  bottom,  ready  for  the  bite.  He 
releases  the  port  friction,  grips  with  the  starboard  and 
partly  opens  the  throttle.  The  closing  wire  is  thus  stressed 
and  the  bucket  closes  upon  its  load  and  rises.  Keeping 
the  load  on  the  closing  wire  and  controlling  the  resulting 
boom  swing  to  starboard  by  a  lesser  backing  strain  on  the 
port  wire,  the  operator  lifts  the  bucket  up  over  the  side 
of  the  scow  and  pocket  coaming  until  it  is  suspended  above 
the  pocket,  when  he  closes  the  throttle,  holds  fast  the  port 
wire  and  releases  the  starboard,  opening  the  bucket. 
Still  gripping  with  his  port  friction,  he  opens  the  throttle 
partially,  swinging  the  boom  to  port  and  then  lowers  it 
open  into  the  water  as  before  to  take  another  bite.  When 
the  pocket  is  fully  loaded,  he  signals  by  blowing  the  whistle 
to  the  deck  hands  who  haul  the  scow  aft  by  operating  the 


28  DREDGING  ENGINEERING 

scow-line  drums  until  the  next  pocket  is  in  position  for 
loading.  The  scow,  settles  more  and  more  deeply  in  the 
water  as  the  loading  progresses,  constantly  decreasing  the 
necessary  height  of  bucket  lift. 

If  dredging  in  tide-water,  the  operator  must  know  the 
stage  of  the  tide  so  that  he  may  dig  the  depth  specified, 
referred  to  the  datum,  usually  mean  low  water.  A  tide 
guage,  therefore,  is  set  where  he  can  see  and  read  it.  In 
addition  to  actul  lead-line  soundings  over  the  bow  of  the 
dredge,  he  is  guided  in  his  digging  by  his  knowledge  of  the 
overall  height  of  the  bucket  or  by  graduations,  or  a  single 
mark  upon  one  of  the  bucket  wires  or  chains  or  upon  the 
bucket  poles,  or  by  means  of  a  wire  leading  from  the  bucket 
through  sheaves  in  the  boom  end  and  "A"  frame  thence 
through  reduction  tackle  to  a  weight  sliding  on  a  graduated 
scale  in  the  pilot  house. 

When  specified  depth  has  been  made  over  the  area  within 
reach  of  the  bucket,  the  dredge  is  moved  ahead  or  "  ad- 
vanced in  the  cut."  If  the  machine  is  operating  under 
wire  control,  this  movement  is  accomplished  simply  by 
winding  in  the  forward  quarter  lines  and  slacking  the  stern 
quarters  and  stern  line.  If  spuds  alone  are  being  used  for 
position  control,  the  advance  is  achieved  by  so-called 
"walking  on  the  spuds,"  as  follows:  The  bucket  is 
grounded,  i.  e.,  lowered  into  the  mud  and  the  stern  and 
one  bow  spud  are  raised  clear  of  the  bottom.  The  operator 
then  stresses  that  bucket  wire  which  tends  to  swing  the  boom 
toward  the  side  on  which  the  bow  spud  is  up,  but  the  boom 
is  anchored  by  the  grounded  bucket  and  the  dredge  is  free 
to  pivot  about  the  third  spud,  so  that  the  boom  retains  its 
position  and  the  dredge  swings.  When  the  free  bow 
corner  has  thus  been  pulled  forward  the  desired  distance, 
that  spud  is  dropped  and  the  operation  repeated  for  the 
advance  of  the  other  bow  spud,  after  which  the  stern  spud 
is  dropped  and  digging  is  resumed.  When  all  pockets 
of  the  scow  are  full,  the  operator  blows  for  the  tug  to  bring 
a  light  scow  and  to  remove  the  loaded  one. 


CHAPTER  III 
.  DIPPER  DREDGES 

General  Description. — As  has  been  said,  the  Dipper 
Dredge  ' '  digs ' '  like  the  familiar  steam  shovel.  The  bucket, 
which  is  essentially  a  scoop  with  heavy  teeth  and  a  flap 
bottom,  is  attached  to  the  end  of  the  dipper  stick,  which 
is  carried  by  the  boom  at  or  near  the  centre  of  the  latter. 
The  boom  is  supported  by  "A"  frame  and  back  guys  as  in 
the  grapple,  except  that,  frequently,  the  "A"  frame  is 
tilted  forward,  when  it  is  termed  the  "  shear  legs."  The 
boom  is  necessarily  of  heavier  construction  than  that  of  the 
grapple  and,  with  the  "A."  frame,  is  set  well  forward  at 
the  very  bow  of  the  hull. 

The  bucket  obtains  its  load  by  digging  into  the  bottom 
under  the  impetus  of  dipper  stick  and  bucket  wire,  which 
runs  over  a  sheave  in  the  boom  head  and  thence  to  the  main 
engine.  The  boom  is  swing  in  one  of  two  ways;  either  as 
in  the  grapple  by  means  of  two  bucket  wires,  or  by  bull 
wheel  and  swinging  engine,  the  latter  being  the  more 
common.  The  hull  is  stiffer  and  the  spuds  heavier  than 
those  of  the  grapple,  because  of  the  greater  strains.  The 
dredge  is  held  in  position  fcy  three  spuds,  and  advances  by 
grounding  her  dipper  and  stressing  the  backing  chain. 
As  a  rule,  the  crew  is  quartered  on  the  dredge.  The  scows 
are  handled  as  on  grapples. 

The  Bucket. — The  bucket  is  an  open  steel  cylinder, 
provided  with  a  hinged  flap  bottom,  a  handle  or  bail  and 
a  reinforced  cutting  edge  or  lip,  to  which  teeth  are  attached. 
Fig.  9  is  a  picture  of  a  Bucyrus  Dipper  Dredge,  swinging  a 
6  cubic  yard  bucket. 

The  bucket  hoisting  wire  is  made  fast  to  the  centre  of 
the  bail,  and  the  backing  chain,  which  draws  bucket  and 
dipper  stick  back  toward  the  hull,  is  fastened  either  to  the 

29 


30 


DREDGING  ENGINEERING 


DIPPER  DREDGES  31 

rear  face  of  the  bucket  or  to  the  dipper  stick  near  the 
bucket.  The  bottom  door  or  floor  of  the  bucket  is  held 
closed  by  a  latch,  which  locks  automatically  by  virtue  of 
its  bevelled  end  when  the  door  is  forced  upward  and  closed 
by  the  water  pressure,  The  latch  is  drawn  back  to  open 
the  door  by  means  of  a  line  extending  from  it  to  a  lever  at  a 
point  near  the  fulcrum,  and  a  second  line  from  the  end  of 
the  lever  to  the  cranesman  stationed  at  the  heel  of  the 
boom.  In  the  larger  buckets  the  latch  rests  on  roller  bear- 
ings to  facilitate  its  movement  and,  in  the  recent,  large 
capacity,  high  powered  machines,  is  steam  operated.  The 
teeth  are  usually  from  three  to  five  in  number,  with  tool 
steel  points,  and  are  detachable  for  sharpening  and  renewing. 

Boom,  Dipper  Stick,  "A"  Frame  and  Back  Guys. — The 
boom,  as  in  the  grapple,  is  hung  from  the  UA"  frame  by 
a  fixed  topping  fall,  but  is  usually  at  a  flatter  angle  with 
the  horizontal  than  that  of  the  grapple.  The  "A"  frame, 
when  vertical,  is  approximately  in  the  plane  of  the  heel 
of  the  boom,  but,  when  inclined,  the  point  of  bearing 
on  the  deck  may  be  some  distance  aft  of  the  boom  heel. 
For  the  same  topping-fall  stress,  the  stresses  in  "A" 
frame  and  back  guys  are  greater  in  the  case  of  the  inclined 
shear  legs  than  in  that  of  the  vertical  "A."  frame.  The 
boom,  necessarily,  is  set  well  forward  in  order  that  the 
dipper  stick  may  clear  the  bow  in  all  positions.  The 
back  stays,  more  particularly  in  those  dredges  having 
inclined  "A"  frames  or  shear  legs,  are  frequently  tension 
members  only,  without  back  legs  or  struts.  There  is  a 
structural  economy  in  the  vertical  "A"  frame,  in  that  it 
and  the  gallows  frame  may  be  designed  as  a  unit  frame, 
whereas  the  inclined  "A"  frame  necessitates  a  distinct 
and  independent  gallows  frame. 

The  boom  is  the  most  difficult  part  of  the  dredge  to 
design.  Some  of  the  very  heavy  loads  to  which  it  is  sub- 
jected are  indeterminate,  principally  those  caused  by 
starting  to  swing  the  boom  before  the  bucket  is  clear  of  the 
water,  or  even  while  still  in  the  mud,  and  by  sudden  stop- 
page and  reversal  of  swing.  The  principal  boom  stresses 


32 


DREDGING  ENGINEERING 


are  again  combined  compression  and  bending,  but,  in  this 
instance,  the  bending  is  due  to  live  as  well  as  dead  load 
and  is  very  much  greater.  In  addition  there  is,  in  those 
booms  swung  by  a  bull  wheel  at  the  heel,  a  horizontal  bend- 
ing movement,  caused  by  the  rotation  of  the  bull  wheel 


Fig.  B 


FIG.   10. — A,  Forces  acting  at  the  dipper  for  two  positions  of  the  arm.     B,  stress 
diagrams  for  the  two  positions. 

and  maximum  at  the  boom  heel  casting.  Furthermore, 
the  side  thrust  of  the  dipper  stick  requires  considerable 
lateral  boom  stiffness.  ^To  investigate  understandingly 
the  vertical  bending  stress  in  the  boom,  a  knowledge  of 
the  action  and  control  of  the  dipper  stick  is  necessary. 


DIPPER  DREDGES  33 

The  stiek  has  two  movements,  one  of  translation  with 
respect  to  and  through  the  boom  at  or  near  its  mid  point, 
and  the  other  of  rotation  in  a  vertical  plane  through  an  arc 
centred  at  the  same  point.  Referring  to  Fig.  10,  the 
dipper  may  be  pulled  either  toward  the  end  of  the  boom  by 
the  main  engine  wire  A,  or  back  toward  the  hull  by  the 
backing  chain  B.  The  motion  of  the  stick  through  the 
boom  is  controlled  by  two  band  friction  wheels  C  on  the 
boom,  keyed  to  a  shaft  carrying  a  pinion  which  meshes  into 
a  rack  on  the  under  side  of  the  dipper  stick,  so  that  the 
stick  may  be  held  fast  at  the  boom  at  any  point  of  its 
length,  at  the  same  time  being  free  to  rotate  about  that 
point.  The  stick  is  pulled  up  through  the  boom  by  the 
tension  in  hoisting  wire  or  backing  chain,  or  both,  and  drops 
down  through  the  boom  by  gravity  alone.  Many  dippers, 
however,  are  equipped  with  a  so-called  "crowding  engine/7 
which  is  mounted  on  the  boom  and  drives  the  pinion 
meshing  with  the  rack  of  the  stick,  so  that  the  stick  can  be 
pushed  or  pulled  through  the  boom.  By  this  means,  the 
dipper  can  be  thrust  out  beyond  the  end  of  the  boom  to  an 
increased  reach. 

It  is  apparent  from  the  stress  diagrams  (fig.  10-b)  that 
the  digging  power,  or  the  thrust  at  the  bucket  perpendicular 
to  the  dipper  stick,  varies  inversely  as  the  length  L  of 
stick  below  the  boom  and  the  angle  6  between  boom  and 
stick;  and  that  th.e  compression  in  the  dipper  stick  is 
maximum  when  L  and  6  are  maximum.  This,  then,  is 
the  critical  loading  of  the  stick  and,  knowing  the  greatest 
pull  of  which  the  main  engine  is  capable,  the  maximum 
comprssive  stress  in  the  stick  follows.  Its  length  will 
usually  be  such  as  to  necessitate  the  use  of  long  column 
formula  in  its  design. 

It  is  entirely  possible,  too,  that  the  dipper  stick  be  sub- 
jected to  tension,  which,  although  insufficient  to  influence 
the  choice  of  section,  is  enough  to  require  a  test  of  the  stick 
details  for  resistance  to  the  tensile  stress  involved,  which 
will  be  the  weight  of  the  loaded  bucket  in  air.  This  condi- 
tion obtains  when,  through  faulty  operation,  or  the  parting 


34 


DREDGING  ENGINEERING 


DIPPER  DREDGES  35 

of  the  main  hoisting  wire,  or  slipping  main  engine  frictions, 
the  full  load  of  the  bucket  and  contents  is  suspended 
from  the  boom  by  the  dipper  stick,  held  by  its  frictions  at 
the  boom.  If  there  be  a  crowding  engine,  the  same  ten- 
sion may  easily  exist  through  the  agency  of  that  engine. 
In  addition,  there  is  a  certain  amount  of  twisting  to  be 
resisted. 

The  direct  stress  in  boom  and  topping  fall  may  be  found 
graphically  as  in  fig.  12.  The  forces  be  and  ac,  fig.  12-B, 
representing  the  bucket  wire  tension,  are  obviously  equal 
in  amount,  as  are  ac  and  de  of  fig.  12-C,  because  the  wire 
passes  over  a  sheave  in  the  boom  head,  and  are  measured  by 
the  pull  of  the  main  engine.  The  boom  compression  is 
maximum  when  ac  is  perpendicular  to  cd,  and  the  topping 
fall  tension  cd  increases  with  the  angle  0.  It  must  be  re- 
membered in  the  design  of  the  latter,  that  the  angle  9  is  not 
limited  by  the  vertical  position  of  the  bucket  wire,  as  the 
bucket  may  easily  be  thrust  out  beyond  the  end  of  the  boom, 
more  especially  by  the  use  of  a  crowding  engine.  The 
dead  load  stress  in  the  topping  fall  is  obtained  as  described 
under  grapples. 

There  are  four  values  of  bending  moment  to  be  con- 
sidered in  the  boom:  a, -the  positive  vertical  B.M.  caused 
by  the  suspension  of  the  loaded  bucket  and  dipper  stick 
as  mentioned  above;  6,  the  negative  vertical  B.M.,  caused 
by  the  thrust  of  the  dipper  stick  when  perpendicular  to 
the  boom;  c,  the  positive  dead  B.M.  due  to  the  weight  of 
the  boom  itself;  and  d  the  horizontal  B.M.  as  a  result  of  the 
bull-wheel  rotation.  The  critical  condition  of  stress  in 
the  boom,  then,  is  the  greatest  possible  combination  of 
direct  compression  and  compressive  fibre  stress  due  to 
vertical  and  horizontal  bending  moment,  not  omitting 
to  investigate  and  provide  for  the  fibre  tension.  These 
stresses,  in  conjunction  with  those  due  to  the  indetermi- 
nate loads  previously  referred  to  and  with  the  physical 
considerations  influencing  the  shape  and  location  of  some 
of  the  members,  result  in  a  sort  of  compromise  design,  a 
and  b  are  maximum  at  the  point  of  intersection  of  dipper 


36 


DREDGING  ENGINEERING 


stock  and  boom;  c  at  the  centre  of  the  boom;  and  d  at  the 
heel  casting.  The  horizontal  B.M.  d  is  equal  to  the  pull 
of  the  swinging  engine  multiplied  by  the  radius  of  the  bull 
wheel.  It  has  little  bearing  upon  the  choice  of  the  boom 


FIG.    12.- — A,    The    purchase-rigged    dipper.     B,  boom-end    forces    and    stress 
diagram  for  purchase-rigged  dipper.  C,  the  same  for  the  direct-wire  dipper. 

section,  but  is  an  important  destructive  agent  acting  upon 
the  heel  casting  and  the  boom  structure  immediately 
adjacent  thereto,  and  one  warranting  complete  investiga- 
tion and  the  provision  of  ample  resistance. 


DIPPER  DREDGES  37 

Some  dippers  are  rigged  with  a  single  sheave  purchase  in 
the  bucket  wire,  fig.  12-A;  i.e.  the  end  of  the  wire  is 
attached  to  the  boom  end,  from  whence  it  passes  down  to 
a  single-sheave  block  fastened  to  the  bail  of  the  bucket, 
thence  up  over  the  boom-head  sheave  and  to  the  main 
engine.  It  is  apparent  that  the  result  of  such  a  rigging  is 
to  double  the  lifting  power  of  the  engine  and  to  halve  the 
lifting  speed  and  that,  in  dippers  of  the  same  digging  power, 
the  boom  stress  in  the  machine  which  is  purchase  rigged 
will  be  less  than  that  in  the  direct  wire  dredge  (Fig.  10). 

The  boom  is  a  relatively  large  member,  having  considera- 
ble depth  at  the  dipper  stick  and  tapering  toward  both 
ends,  and  is  of  plate,  girder  or  latticed  truss  construction. 

The  stress  in  "A"  frame  or  shear  legs  and  back  legs  or 
stays  are  found  as  for  grapples  in  Chapter  I. 

Spuds. — The  two  bow  spuds  are  set  either  in  "outside" 
or  " through"  wells.  They  are  of  heavy  section  and  are 
subject  to  considerable  bending  moment  occasioned  by  the 
reaction  of  the  forward  thrust  of  the  dipper.  Some  dippers 
are  rigged  to  "pin  up,"  a  term  applied  to  the  process  of 
maintaining  the  dredge  upon  an  even  keel  while  digging, 
by  transferring  part  of  the  weight  of  the  machine  to  the 
two  bow  spuds,  so  that  it  is  not  dependent  upon  the  sta- 
bility of  the  hull  to  resist  transverse  oscillation.  In  pin-up 
dredges,  the  fore  spuds  are  provided  with  sheaves  both 
top  and  bottom.  A  wire  attached  to  the  spud-well  housing, 
running  down  around  the  toe  sheave  and  thence  up  to  the 
drum  of  the  spud  hoist,  raises  the  spud,  and  a  second  wire 
of  larger  diameter,  fastened  at  the  same  place  and  leading 
up  over  the  top  sheave  and  down  to  the  drum,  becomes 
taut  when  the  dredge  attempts  to  list  to  that  side,  tending 
to  force  the  spud  more  deeply  into  the  bottom. 

The  dredge  advances  by  grounding  her  bucket  well 
ahead  of  the  bow,  raising  the  two  bow  spuds  clear  of  the 
bottom  and  stressing  the  backing  chain,  which  pulls  the 
machine  toward  the  dipper.  The  stern  spud  remains  in  the 
mud  during  the  operation,  and,  having  a  slotted  well, 
slowly  inclines  forward  as  the  dredge  progresses.  It  is 


38 


DREDGING  ENGINEERING 


DIPPER  DREDGES  39 

called  the  "  walking-spud "  or  "  trailing-spud  "  on  this 
account.  The  spuds  are  raised  by  spud  hoist  and  gallows 
frame,  or  by  a  rack  on  the  spud  engaging  a  pinion  at  the 
deck  level,  obviating  the  necessity  of  a  gallows  frame. 
The  Machinery. — In  dipper  dredges  having  two  bucket 
wires  to  swing  the  boom  and  raise  the  bucket,  the  main 
engine  is  a  two  cylinder  horizontal  type,  with  two  drums 


FIG.   14. — Dipper  dredge  with  bank  spuds.     (Courtesy  the  Marion  Steam  Shovel 

Co.) 

as  in  the  grapple.  If  the  boom  is  controlled  by  swinging 
engine  and  bull  wheel,  the  main  engine  may  have  but  one 
hoisting  drum.  In  some  machines,  the  drum  cylinder 
is  of  large  diameter  for  part  of  the  width,  stepping  down  to  a 
smaller  diameter  for  the  balance,  the  object  being  to  in- 
crease the  pull  on  the  bucket  wire  during  the  early  stage 
of  digging  when  the  bucket  is  loading,  after  which,  the  wire 
coils  upon  the  larger  drum  surface,  accelerating  the  raising 
of  the  dipper.  Such  a  device  is  termed  a  differential  drum, 
and  is  used  to  good  advantage  with  the  purchase  rigged 


40  DREDGING  ENGINEERING 

dipper.  In  addition  to  the  main  and  swinging  engines, 
complete  dipper  control  requires  an  engine  for  handling 
the  backing  chain  which  draws  the  bucket  back  toward 
the  hull.  Some  machines  are  equipped  also  with  a  crowd- 
ing engine  mounted  upon  the  boom,  with  the  function  of 
dipper  stick  motion  as  previously  described.  The  mach- 
inery for  spud  and  scow  handling  is  similar  to  that  of  the 
grapple. 


FIG.   15. — The  Tellico — U.    S.   Army  Engineers — 1M   yd.   dipper,   triple  hitch. 
(Courtesy  the  Osgood  Co.) 

The  dredge  PRESIDENT,  American  Dredging  Company, 
pictured  on  page  38,  is  a  "pin-up"  machine,  with  direct 
wire  rigging,  bull  wheel  and  crowding  engine,  hull  105  ft. 
X  39  ft.  8  in.  X  10  ft.  7  in.,  2  cyl.  hor.  main  engine  14  X 
16,  and  a  4>^  yd.  dipper. 

The  Hull. — In  general  dimensions  and  construction 
details,  the  hull  is  very  similar  to  that  of  the  grapple. 
There  is,  howrever,  a  need  for  greater  stiffness  due  to  the 
thrust  of  the  dipper  and  the  extreme  forward  mounting 
of  the  boom.  In  some  types,  e.g.,  the  Bucyrus  built 
machines,  the  longitudinal  trusses  are  steel  and  of  great 


DIPPER  DREDGES  41 

depth,  reaching  from  the  cross  keelsons  to  the  roof  of  the 
deck  house,  and  tied  horizontally  by  a  top  lateral  truss. 
There  is,  too,  relatively  less  stability  forward  than  in 
grapples  to  resist  listing  when  the  bucket  is  swung  out  over 
the  scow,  from  which  it  is  apparent  that  the  same  hull  will 
carry  a  grab  bucket  of  larger  capacity  than  a  dipper. 
The  bow  structure  of  the  hull  must  be  well  stiffened  and 
the  truss  and  superstructure  design  there  is  dependent  in 
some  measure  upon  the  locating  of  the  bull  wheel,  which 
may  be  upon  the  deck  or  elevated  to  the  plane  of  the  roof 


FIG.   16. — 14"  X  16"  engines  and  hoisting  machinery  for  a  6  yd.  dipper  dredge. 
(Courtesy  the  Bucyrus  Co.) 

of  the  deck  house.  In  the  latter  position,  it  exerts  a  more 
direct  pull  upon  the  boom,  thereby  reducing  the  lateral 
stresses  in  that  member. 

Operation. — Dippers  require  two  men  to  operate  the 
boom,  dipper  stick  and  bucket,  in  addition  to  the  usual 
crew  of  engineer,  firemen,  oilers  and  deck  hands.  One, 
the  " operator"  or  " runner,"  controls  the  bucket  hoist, 
backing  chain  and  boom  motion  through  the  throttles 
and  frictions  of  the  main,  backing  and  swinging  engines. 
The  other,  the  "cranesman"  or  "  dipper  tender,"  is  sta- 
tioned at  the  boom  heel  and  regulates  the  dipper  stick 
frictions  on  the  boom  and  opens  the  bucket. 

In  the  process  of  digging,  the  runner  slacks  the  bucket 
wire  or  wires,  permitting  it  to  drop  into  the  water,  at  the 


42 


DREDGING  ENGINEERING 


same  time  pulling  it  back  toward  the  hull  by  stressing  the 
backing  chain.  The  cranesman  releases  the  dipper  stick 
frictions  so  that  it  falls  through  the  boom  until  the  dipper 
rests  on  the  bottom.  The  runner  releases  the  backing 
chain  and  stresses  the  hoisting  wire,  and  the  dipper  tender 
applies  his  friction  bands,  gripping  the  dipper  stick  at  the 
boom.  The  bucket,  therefore,  cuts  its  way  forward  in  an 
arc  of  radius  equal  to  that  portion  of  the  dipper  stick  below 
the  boom  until  it  is  loaded  and  clear  of  the  mud,  whereupon 


FIG.   17. — Hoisting  and  Backing  Drums — 14"  X  16"  dipper  dredge.      (Courtesy 

the  Bucyrus  Co.) 

the  cranesman  releases  the  stick,  which  shoots  up  through 
the  boom.  The  runner  swings  the  boom  until  the  bucket 
is  suspended  over  a  pocket  of  the  scow  and  the  cranesman 
pulls  the  latch  string,  dumping  the  bucket. 

Application  of  the  Type. — To  the  need  of  the  American 
contractor  for  a  simple,  inexpensive  and  versatile  machine, 
the  dipper  dredge  owes  its  rapid  development  in  this 
country.  It  is  very  efficient  in  hard  material,  provided 
the  depth  is  not  excessive,  and  is  used  to  advantage  in 
canal  work  through  solid  ground  containing  stumps  and 
roots,  in  the  dredging  of  previously  blasted  or  loose  rock 


DIPPER  DREDGES 


43 


44  DREDGING  ENGINEERING 

and  in  the  removal  of  old  filled  cribs,  old  foundations, 
sunken  wrecks  and  stone  dikes.  Thus  it  is  a  most  capable 
machine,  with  a  wide  application.  The  conditions  of 
uniformly  hard  materials  and  moderate  requisite  depth  of 
channel  on  the  Great  Lakes  have  occasioned  an  extensive 
use  of  the  dipper  there. 

High-powered  Dipper  Dredges. — The  earliest  dipper 
dredge  of  large  size  was  the  12  yard  machine  ONONDAGA, 
owned  and  operated  in  New  York  Harbor  about  1904 
by  the  contractors  Hughes  Brothers  &  Bangs.  Since  then, 
and  prior  to  1904,  the  largest  exponents  of  the  type  were 
probably  the  two  10  yard  machines  used  on  the  Cape  Cod 
Canal,  built  by  the  Atlantic  Equipment  Co.,  and  the  15 
yard  dredge  TOLEDO,  built  by  the  Bucyrus  Company. 

At  the  time  of  the  closing  of  the  Panama  Canal  by  the 
first  large  slide,  the  largest  dippers  in  use  there  were  5 
yard.  It  was  decided  that  the  only  type  of  machine  suita- 
ble for  the  removal  of  this  huge  mass  of  broken  rock  and 
earth,  containing  pieces  of  rock  of  all  sizes  and  lacking 
all  uniformity  of  formation,  was  a  large  capacity  and  high- 
powered  dipper  dredge.  The  decision  resulted  in  the  con- 
struction by  the  Bucyrus  Company  of  three  15  yard  dipper 
machines,  the  GAMBOA;  PARAISO  and  CASCADAS,  the  latter 
being  the  last  built  and  arriving  at  the  Isthmus  in  October 
of  1915.  Although  open  to  criticism  perhaps  as  to  the 
abnormally  high  cost  of  maintenance,  attributed  by  some 
engineers  to  defects  in  the  main  hoisting  wires,  the  dipper 
arms  and  the  spuds,  both  the  builders  and  the  management 
were  justified  by  the  splendid  performance  of  the  machines 
and  by  the  fact  that  the  first  two  were  in  large  part  copies 
of  the  TOLEDO  because  of  the  lack  of  time  to  prepare  new 
designs.  Each  of  the  three  dredges  had  a  demonstrated 
capacity  of  more  than  3,000,000  cubic  yards  per  year. 
They  are  all  pin-up  machines,  with  steel  hulls.  The  princi- 
pal statistics  of  the  above  dredges  are  given  in  the  Table, 
page  45. 


DIPPER  DREDGES 


45 


ONONDAGA  GAMBOA  &  PARAISO 

Hull  length 140'  144' 

width 50'  44' 

depth 15'  13'-6" 

Main  Engines — Type Double  cylinder  Compound 

condensing 

Size 20".5  X  24"  16  and  28  X  24 

Forward  Spuds — Material   .  .                Oregon  Fir  Structural  Steel 

Section    ...                  5'  X  5'  4'  X  4' 

Length 80'  82' 

Dipper  Arm — Material Oregon  Fir  Long  Leaf  Y.  P. 

Section 3'  X  3' 

Length 80'  72' 

Capacity  of  Dipper 15  cu.  yd.  Rock  10  cu.  yd. 

Mud  15  cu.  yd. 

Max.  Working  Depth 50'  50' 

Bail  Pull 235,000  Ibs. 

N.  B.  The  CASCADAS  differed  principally  in  the  width  of  hull,  which 
was  55  ft.,  in  the  depth,  15'-6",  and  in  the  use  of  a  gallows  frame  to  raise 
the  spuds  without  the  use  of  sheaves  in  the  spud  toes,  which  proved 
objectionable  because  of  the  cable  abrasion  due  to  the  rock.  Several 
improvements  were  also  made  in  the  machinery  and  in  other  details  as  dic- 
tated by  the  experience  with  the  other  two. 


CHAPTER  IV 
LADDER  DREDGES 

Historical. — The  ladder  or  elevator  dredge  is  distinctly 
a  European  product  and,  until  recently,  has  found  little 
favor  in  the  United  States  for  major  dredging  projects. 
The  reason  lies  not  so  much  in  the  qualities  of  the  dredge 
itself,  but  rather  in  the  quite  opposite  methods  of  admin- 
istration of  port  development  of  the  two  continents.  Mr. 
A.  W.  Robinson,  in  his  paper  on  the  "  Review  of  General 
Practice,"  printed  in  Vol.  LIV  of  the  Transactions  of  the 
American  Society  of  Civil  Engineers,  1905,  writes:  "In 
America,  the  opening  up  of  a  vast,  virgin  country,  both  on 
the  seaboard  and  in  the  interior,  required  the  rapid  execu- 
tion of  a  great  number  of  works  suited  to  immediate  neces- 
sities, under  the  small  contract  system.  This  system  has 
given  rise  to  a  class  of  contractors  of  all  grades,  who  build 
and  own  their  plant,  and  as  their  contracts  are  liable  to 
be  varied  both  as  to  locality  and  conditions,  they  do  not 
invest  largely  in  special  plant.  The  small  contractor 

.  .  must  employ  a  plant  which  is  adapted  to  a  variety 
of  work,  and  which  does  not  represent  more  capital  in- 
vested than  his  contract  will  warrant.  ...  On  the  other 
hand,  the  large  corporation,  or  Board  of  Harbor  Trustees, 
under  the  European  method  of  administration,  is  able  to 
lay  out  a  comprehensive  plan  of  the  works  under  its  charge 
and  provide  permanent  plant  adapted  to  it,  which  will  be 
assured  of  employment  through  a  series  of  years.  This 
has  developed  the  large  and  complete  seaworthy  dredge 
of  the  ladder  and  bucket  type,  which  is  found  so  frequently 
in  Europe  and  so  rarely  in  America." 

The  inexpensive  and  versatile  dipper  dredge,  therefore, 
was  early  and  rapidly  developed  in  this  country,  proving 
an  excellent  tool  especially  for  harbor  and  shallow  work  as 

46 


LADDER  DREDGES 


47 


48  DREDGING  ENGINEERING 

found  in  the  Great  Lakes,  and  the  grapple  dredge  attained 
equally  rapid  development  on  the  softer  tidal  work  of  the 
Coast,  and  both  to  the  almost  absolute  exclusion  of  the 
ladder  or  elevator  type. 

In  more  recent  years,  however,  the  tendency  has  been 
to  the  design  of  machines  of  larger  capacity,  in  the  belief 
that  their  lower  unit  cost  of  output  justifies  the  greater 
initial  outlay.  This  trend,  together  with  the  attention 
demanded  by  the  ladder  type  in  its  economic  performance 
on  the  Panama  Canal,  has  led  to  a  more  general  acceptance 
in  this  country  of  the  advantages  of  the  elevator  dredge,  as 
is  evidenced  by  the  purchase  in  Scotland  of  a  large  machine 
of  this  type  for  use  on  the  Panama  Canal.  The  capacity  of 
each  of  the  soft-digging  buckets  of  this  dredge  is  2  cubic 
yards,  and  of  the  hard-digging,  35  cubic  feet. 

Moreover,  in  Canada,  on  the  St.  Lawrence  River  Ship 
Channel,  the  ladder  dredge  has  been  in  active  use  for  years, 
even  antedating  the  development  of  the  dipper.  The 
type  secured  a  foothold  in  Canada  from  early  examples 
imported  from  Scotland,  and,  because  of  its  adaptability 
both  to  the  local  dredging  conditions  and  to  the  method  of 
government  operation  through  a  number  of  years,  its 
popularity  there  has  persisted. 

It  is  now  generally  conceded  that  the  very  hard,  indu- 
rated clays,  shales,  soft  rock  formations  and  hard-pans, 
when  excavated  in  their  original  condition  without  pre- 
vious blasting,  are  most  economically  dredged  by  ladder 
machines  and  that,  in  hard  clays,  the  choice  between  the 
ladder  and  the  dipper  is  difficult,  except  as  determined 
by  depth  of  water  and  depth  of  cut. 

The  ladder  dredge  of  the  so-called  "stationary"  type 
has  found  favor  for  some  years  in  two  American  industries, 
viz.  dredging  for  gold  in  the  western  states  with  the  large 
placer,  elevator  machine,  and  for  commercial  sand  and 
gravel.  The  peculiar  virtues  of  the  type,  to  which  it  owes 
its  superiority  for  gold  dredging,  are  the  clean  pick-up  of 
the  gold  bearing  gravel  with  a  minimum  of  agitation  and 
the  delivery  to  the  screen  of  an  almost  continuous  stream 


LADDER  DREDGES 


49 


50  DREDGING  ENGINEERING 

without  leakage.  The  failure  of  other  types  of  dredge 
in  these  two  essentials,  resulting  in  an  excessive  loss  of 
gold,  has  militated  against  their  employment  in  the 
industry. 

In  Chapter  IX,  under  the  caption  "Choice  of  Plant," 
American  and  Continental  opinion  upon  the  relative  merit 
of  the  ladder  dredge  is  further  cited. 

General  Description/ — The  digging  mechanism  of  the 
ladder  dredge  consists  of  a  long  trussed  ladder,  inclining 
from  the  top  of  a  tower  through  a  slot  or  well  in  the  hull 
down  to  the  mud  line;  a  series  of  buckets  traveling  in 
endless  chain  succession  up  the  top  side  and  down  the  lower 
side  of  the  ladder,  and  carried  by  two  tumblers  or  drums 
at  the  two  extremities  thereof  and  by  rollers  upon  its  upper 
face;  and  the  main  engine  actuating  the  upper  tumbler, 
which  engages  the  endless  bucket  chains.  The  ladder  may 
be  raised  and  lowered,  pivoting  about  the  tower  top.  The 
buckets  obtain  their  loads  by  scraping  along  the  bottom 
in  the  act  of  revolving  about  the  lower  tumbler,  and  dis- 
charge when  they  invert  at  the  upper  or  driving  tumbler, 
which  is  driven  by  the  main  engine  through  chain  or  belt, 
or  more  commonly  through  gears  and  a  friction  clutch. 
This  last  is  intended  to  safeguard  the  engine  against  pos- 
sible sudden  shock  due  to  abrupt  resistance  or  impact  of  the 
buckets.  The  velocity  of  the  bucket  travel  is  varied  to 
meet  the  conditions  of  material  and  depth,  preferably 
through  the  gearing  independent  of  the  main  engine.  It  is 
essential  that  the  dredge  be  equipped  with  two  sets  of 
buckets,  one  for  hard  and  the  other  for  soft  digging.  They 
vary  in  size  generally  from  5  cu.  ft.  to  1  cu.  yd.  capacity. 
The  soft  digging  buckets  are  larger  for  the  same  machine 
than  the  hard  digging,  and  are  re-enforced  on  the  cutting 
edge  with  hard  steel  lips.  The  buckets  designed  for  hard 
material  are  heavily  constructed  and  are  provided  with 
teeth  at  the  lip.  Both  digging  and  discharging  are  facil- 
itated by  tapering  the  buckets  in  longitudinal  section  both 
vertically  and  horizontally. 


LADDER  DREDGES 


51 


52  DREDGING  ENGINEERING 

Stationary  Type.— The  hull  of  the  stationary  type  is 
generally  rectangular  both  in  plan  and  in  cross  section, 
pierced  with  a  well  or  slot  through  which  the  ladder 
operates.  The  feeding  movement  is  either  lateral  through 
the  agency  of  six  mooring  cables  to  as  many  anchorages, 
or  radial,  the  dredge  pivoting  about  a  stern  spud  in  the 
manner  of  the  radial-feeding  hydraulic  machine.  American 
practice  tends  toward  the  latter  method.  The  objection 
to  the  former  and  an  argument  in  favor  of  the  self-propelled 
type  is  the  obstruction  presented  by  the  cables  to  river 
traffic.  The  St.  Lawrence  River  dredges  are  of  the  lateral 
feed  type,  making  a  cut  as  wide  as  750  feet  in  one  operation, 
the  extensive  movement  being  permitted  by  exceptionally 
long  head  lines  of  wire  rope  carried  on  floats  for  a  consider- 
able distance  ahead  of  the  dredge  to  obviate  the  resistant 
dragging  on  the  bottom. 

The  dredged  material  is  conducted  to  scows  moored  to 
the  dredge  through  chutes  leading  from  the  receiving  hopper 
at  the  top  of  the  tower.  Machines  operating  in  a  mixture 
of  sand  and  gravel  for  commercial  usage,  contain  a  cylin- 
drical rotating  screen  at  the  top  which  segregates  the 
material  so  that  sand  is  discharged  into  a  scow  on  one  side 
of  the  dredge  and  gravel  into  a  second  scow  on  the  other 
side.  Some  dredges  have  abnormally  high  towers  and  long 
chutes  supported  from  the  dredge,  depositing  the  dredgings 
at  some  little  distance.  Flow  in  the  chute  pipes  is  some- 
times accelerated  by  pumping  water  through  them.  When 
it  becomes  necessary  to  transport  the  material  to  points 
more  remote  from  the  dredge,  floating  pipe  lines  are 
employed,  through  which  the  material  is  forced  by  water 
from  a  discharge  pump.  In  some  instances,  a  series  of 
belt  conveyors  mounted  on  lighters  has  been  used. 

Sea-going  Hopper  Type. — The  sea-going  ladder  dredge 
is  a  self-propelling  steamer,  with  moulded  hull  and  self- 
contained  hoppers.  When  loaded,  it  travels  to  the  dump- 
ing ground  under  its  own  power  and  discharges  through 
doors  at  the  base  of  the  hoppers.  They  are  usually  of 
the  twin-screw  type,  driven  either  by  the  main  dredging 


LADDER  DREDGES  53 

engine  or  by  an  independent  unit.  It  is  customary  to 
provide  for  discharging  both  into  the  dredge  hoppers  and 
into  hopper  barges  moored  alongside. 

Dredges  of  this  type  vary  through  wide  limits  as  to  size, 
capacity  and  maximum  depth  of  dredging;  in  length  from 
about  130  to  275  feet;  in  hopper  capacity  from  about  500 
to  2200  tons;  and  in  greatest  possible  depth  of  dredging, 
from  about  30  to  50  feet.  The  previously  mentioned  ma- 
chine, purchased  in  Scotland  by  the  Isthmian  Canal  Com- 
mission, has  a  specified  capacity  of  1200  cubic  yards  of  mud 
or  sand  per  hour,  and  a  maximum  dredging  depth  of  50 
feet.  The  speed  of  sea-going  ladder  dredges  is  generally 
from  6  to  8.  miles  per  hour,  when  loaded. 

While  the  stationary  machine  may  have  two  ladders, 
one  on  each  side,  the  sea-going  dredge,  for  reasons  of  sta- 
bility, has  but  one  on  the  longitudinal  axis  of  the  hull. 
The  tower  is  erected  amidships  and  the  well  may  be  either 
forward  or  aft.  There  are  two  types  of  well,  the  close- 
ended  and  the  open-ended.  The  latter  is  more  commonly 
used,  due  to  the  advantage  of  being  able  to  dig  flotation 
for  the  dredge.  General  practice  employs  the  bow  well 
for  single-screw  steamers  and  the  stern  well  for  twin-screw. 
The  well  is  proportioned  to  the  length  of  the  ladder.  In 
the  bow-well  dredge,  the  ladder,  when  raised,  lies  wholly 
within  the  well,  and  a  breakwater  is  installed  at  the  after 
end  of  the  well  to  deflect  the  water  under  the  keel  of  the 
vessel  when  steaming  ahead. 

The  length  of  the  ladder  should  be  such  as  to  reach  the 
maximum  required  depth  when  at  an  inclination  of  45 
degrees,  since  the  maximum  amount  of  work  is  said  to  ob- 
tain at  that  angle.  Some  designers  have  even  gone  so  far 
as  to  build  the  ladder  in  two  parts,  so  that  the  lower  sec- 
tion may  always  have  a  slope  of  45  degrees. 

For  the  sea-going  ladder  dredge,  the  advantage  is  claimed 
of  entire  seaworthiness  in  ocean  dredging  and  the  absence 
in  rough  water  of  the  pounding  of  attendant  plant,  so  de- 
structive both  to  such  plant  and  the  dredge  itself.  It  is 
open  to  the  criticism,  however,  of  intermittent  dredging 
operation  due  to  the  frequent  trips  to  the  dumps. 


CHAPTER  V 
SCOWS 

The  most  common  conveyance  for  the  transportation  of 
dredgings  (pumpings  excepted)  is  the  bottom-dump  mud 
scow.  It  is  in  effect  a  hopper  barge,  without  means  of  self 
propulsion,  consisting  simply  of  a  hull  rectangular  in  plan 
and  cross  section,  containing  a  number  of  independent 
hoppers  called  pockets,  and  with  both  ends  raked  or 
rounded  (in  elevation)  for  towing  in  either  direction. 
Roughly,  it  is  somewhat  more  than  three  times  as  long  as 
it  is  wide,  and  the  depth  of  hull  from  deck  to  bottom  plank- 
ing is  about  one-third  of  the  beam.  When  light,, it  has 
considerable  freeboard,  but  when  fully  loaded,  it  floats 
with  decks  awash,  the  material  being  retained  by  the  pocket 
"  coamings '; — a  name  given  to  the  portions  of  the  pocket 
walls  projecting  above  the  deck.  The  coaming  height  is 
usually  from  two  to  three  feet. 

The  common  form  of  cross  section  is  shown  in  fig.  22. 

Neglecting  the  fore  and  after  holds,  between  the  ends 
of  the  scow  and  the  faces  of  the  first  and  last  pockets, 
the  vessel  is  dependent  for  buoyancy  upon  the  polyhedral 
spaces  between  the  sides  of  the  scow  and  the  sloping  side 
walls  of  the  pockets,  unless  the  continuity  of  the  pockets 
is  broken  by  one  or  two  transverse  holds,  as  is  sometimes 
done,  not  merely  to  increase  the  displacement,  but  also 
to  add  to  the  transverse  strength.  Thus,  the  displacement 
tonnage  per  inch  immersion  decreases  quite  rapidly  as  the 
draft  increases,  and  the  loaded  scow  has  no  reserve  buoy- 
ancy. While  these  two  features  may  appear  objectionable, 
and  while  we  do  hear,  occasionally  of  the  capsizing  of  a 
mud  scow,  yet  the  good  points  of  the  design  are  so  pre- 
ponderant and  the  disaster  of  capsizing  so  trivial  that  the 
popularity  of  the  type  lives  on. 

54 


scows 


55 


It  will  be  remembered  that  mud  scows  are  loaded,  not 
as  the  designer  would  have  them  for  minimum  stresses 
in  the  structure,  but  from  one  end  to  the  other,  and  that 
usually  they  are  dumped  in  the  same  way.  Obviously 
the  resultant  tendency  is  to  "hog"  the  scow,  i.  e.  by  nega- 
tive bending  moment  in  the  structure  as  a  whole,  to 
render  it  convex  upward.  To  provide  adequate  stiffness 
longitudinally,  therefore,  a  pair  of  strong  trusses  are  built 
in  the  plane  of  the  side  coamings  for  the  full  length  and 
depth  of  the  scow.  In  timber  scows,  they  are  generally  of 
the  Howe  type.  Supplementing  them,  the  sides  of  the 


FIG.  22. — Cross  section  of  the  common,  bottom-dump  mud  scow. 

vessel  are  detailed  to  further  resist  the  "hogging"  strains 
in  that  the  upper  strakes  are  in  long  lengths  and  spliced 
with  long  tension  scarf  joints.  The  lower  strakes  are  as  a 
rule  framed  with  long  scarf  joints  without  the  tension 
feature.  The  intermediate  courses  may  be  butted  at 
stanchions.  The  truss  rods  should  be  provided  with  turn- 
buckles,  that  they  may  be  adjusted  to  timber  shrinkage 
and  local  fibre  crushing.  The  transverse  strength  of  the 
scow  is  provided  by  the  walls  between  pockets,  which  are 
solid  bulkheads  extending  the  full  width  of  the  ship. 

The  floor  of  each  pocket  consists  of  a  pair  of  doors, 
hinged  at  the  sides  to  open  downward  at  the  centre.  A 
chain  at  each  end  holds  them  horizontal  or  closed,  running 
through  a  lead  chock  to  a  shaft  just  outside  the  coaming. 


56  DREDGING  ENGINEERING 

Upon  this  the  chain  is  wound  by  lever  bar,  ratchet  wheel 
and  pawl.  Each  pocket  has  its  own  gear  and  is  dumped 
independently  of  the  others,  by  pounding  the  pawl  out  of 
engagement  with  the  ratchet  wheel,  whereat  the  chain  is 
released,  the  doors  fall  open  and  the  load  drops  through. 
Although  not  always  feasible,  it  is  desirable  that  the  depth 
of  well  below  the  door  hinges  be  equal  to  the  width  of  a 
single  door,  in  order  that  the  doors  when  open  may  not 
project  below  the  bottom  of  the  scow,  in  which  position 
they  are  liable  to  damage  in  shoal  water. 

Whatever  the  material  of  which  the  scow  is  constructed, 
it  should  be  strongly  and  durably  designed,  to  withstand 
the  severe  treatment  necessarily  meted  out  to  it.  Although 
steel  has  been  used  in  some  instances,  timber  appears  to 
be  best  adapted  to  the  purpose  and  is  quite  prevalent. 
Current  practice  builds  the  body  of  the  scow  of  long  leaf 
yellow  pine,  of  prime  inspection,  with  corners  and  deck 
strake  of  white  oak.  The  corners  are  armored  with  steel 
plates.  The  scow  is  always  moored  to  the  dredge  with  the 
shaft  and  winding  gear  on  the  side  remote  from  the  dredge, 
so  that  the  danger  of  damage  to  the  gear  shall  be  reduced 
to  a  minimum.  The  coamings  on  the  port  side,  therefore, 
are  fitted  with  eye-bolts  at  each  pocket  to  receive  the  hook 
of  the  breast  line  from  the  dredge.  Consequently,  it  is 
the  port  coaming  and  the  port  side  of  the  hull  that  wear 
most  rapidly,  due  to  the  destructive  impacts  of  the  bucket. 
For  this  reason,  the  port  side  is  not  uncommonly  sheathed 
with  2  or  3  inch  oak  or  pine,  preferably  the  former.  Some- 
times the  two  ends  are  protected  likewise,  and  in  teredo- 
infested  waters,  such  sheathing  on  all  sides  is  of  paramount 
importance.  The  scow  is  equipped  with  towing  bitts,  and 
with  hatches  for  ventilation  and  siphoning  out  fore  and  aft. 

For  river  and  harbor  work,  scows  of  500  or  600  yards 
capacity  are  the  .most  popular.  Larger  sizes  present  the 
objectionable  features  of  great  freeboard  when  light, 
requiring  an  inordinately  high  bucket  lift,  and  great  draft 
when  loaded,  requiring  deep  water  for  dumping.  The 
latter  becomes  an  important  factor  in  dumping  to  a  hy- 


SCOWS  57 

draulic  machine  for  pumping  ashore.  In  this  instance, 
the  so-called  rehandling  basin,  or  rectangular  hole  exca- 
vated by  the  pumps  to  receive  the  dumped  material,  is 
located  preferably  as  close  to  the  impounding  basin  as  the 
consideration  of  minimum  depth  for  dumping  scows  will 
permit,  the  idea  being  of  course  to  reduce  the  length  of 
discharge  pipe  line  to  a  minimum.  Hydraulic  dredges  en- 
gaged in  rehandling  work  are  required  to  maintain  a  depth 
in  the  rehandling  basin  at  least  as  great  as  the  surrounding 
depth,  in  order  that  as  little  material  as  possible  shall  be 
lost  between  dumping  and  pumping.  The  limits  of  the  re- 
handling  basin  are  defined  by  ranges  to  control  the  operation. 

Scows  of  types  other  than  the  bottom-dumps  are  in  less 
common  use  for  handling  dredgings.  For  rock  and  com- 
mercial sand  and  gravel,  deck  lighters  are  employed. 
Particular  attention  must  be  given  here  to  the  design  of  the 
deck  for  the  heavy  loads  and  impacts.  For  rock,  a  layer 
of  sheathing  is  usually  laid  on  the  deck  planking.  Con- 
crete deck  scows  have  been  successfully  used  for  sand  and 
gravel.  Deck  lighters  may  be  converted  into  mud  scows 
by  erecting  on  the  deck  a  series  of  gable  bottom  bins,  with 
the  ridge  in  the  plane  of  the  longitudinal  axis  of  the  hull, 
and  discharging  through  vertical  doors  on  both  sides.  Such 
an  arrangement  is  particularly  adaptable  to  shoal  water, 
although  the  centre  of  gravity  of  the  loaded  scow  is  abnor- 
mally high,  and  dumping  at  times  proves  a  rather  delicate 
operation.  A  second  method  of  rigging  deck  scows  to  dis- 
charge their  loads  is  by  the  use  of  a  sliding  deck  platform 
mounted  on  rollers  on  a  slight  incline.  This  false  deck 
when  loaded  and  unlatched  slides  to  one  side  by  gravity 
for  a  distance  of  several  feet,  when  it  is  checked  by  a  stop. 
The  momentum  of  the  load  and  its  eccentric  position  cause 
the  scow  to  list  until  the  material  slides  off  into  the  water, 
whereupon,  the  scow,-  released,  rights  itself. 

For  towing  to  sea,  large  dump  scows,  upwards  of  2000 
yards  capacity,  have  been  used,  and  in  some  instances 
self-propelled  hopper  barges  have  been  employed  to  trans- 
port dredgings. 


CHAPTER  VI 
HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 

As  outlined  in  Chapter  I,  there  are  two  general  and 
distinct  classes  of  hydraulic  dredges,  the  River  Type  and 
the  Sea-going  Hopper  Dredge.  The  first  is  the  smaller 
machine,  built  for  use  in  the  calm  waters  of  rivers  and 
sheltered  harbors,  and  is  rarely  self-propelling  except 
among  those  designed  by  the  Mississippi  River  Commission 
to  meet  the  special  and  unique  conditions  obtaining  upon 
that  mutinous  water-course.  The  second  is  the  more 
imposing,  ocean-going  steamer,  with  moulded  hull  and, 
in  the  great  majority  of  cases,  with  self-contained  hoppers 
for  receiving  and  transporting  the  pumpings.  Although 
at  times  economically  adaptable  to  ship-channel  work  in 
rivers  and  harbors,  it  is  primarily  intended  for  the  removal 
of  obstructive  ocean  bars.  * 

The  type  of  dredge  developed  by  the  Mississippi  River 
.Commission  under  the  direction  of  the  U.  S.  Government, 
embodies  many  distinctive  features,  by  virtue  of  which 
it  has  been  isolated  in  the  foregoing  classification  and  will 
be  discussed  subsequently  in  this  chapter.  The  text 
immediately  following  refers  only  to  the 

RADIAL-FEEDING  DREDGE  WITH  SPUD  ANCHORAGE 

General  Description.— Briefly,  it  consists  of  a  centrifugal 
pump  directly  connected  to  a  steam  engine  or  a  motor 
and  mounted  on  a  hull  equipped  with  a  hinged  ladder, 
carrying  both  suction  pipe  and  cutter-head,  and  with  some 
means  of  position  control  and  feeding  movement. 

The  dredge  may  be  said  to  have  three  principal  functions; 
first,  the  breaking  up  or  cutting  of  the  bottom  material  so 
that  it  readily  may  be  drawn  into  the  suction  pipe;  second, 
the  horizontal  and  vertical  movement  of  the  suction  pipe 

58 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  59 

and  cutter  so  that  the  material  may  be  fed  constantly  to 
the  suction;  third,  that  of  pumping  the  material  through 
suction  pipe,  pump  and  discharge  pipe  into  the  spoil  basin 
or  the  adjacent  deep  water.  The  first  is  accomplished 
by  means  of  a  rotating  cutter  at  the  suction  end  of  the 
ladder,  keyed  to  a  shaft  carried  on  the  ladder  and  driven 
by  an  engine,  called  the  cutter  engine,  located  at  the 
ladder  hinge.  The  vertical  movement  of  the  second  func- 
tion is  obtained  by  boom,  "A"  frame,  back  guys,  fall  and 
hoisting  engine,  by  which  the  ladder  is  raised  and  lowered. 


FIG.  23. — The  Pennsylvania — American  Dredging  Co. 

The  horizontal  movement,  laterally,  is  effected  in  one  of 
two  ways;  either  by  swinging  the  dredge  and  ladder  as  a 
whole  about  one  of  the  two  stern  spuds  as  a  pivot,  or  by 
swinging  the  ladder  only,  the  dredge  being  held  by  three 
or  four  spuds.  The  forward  movement  or  the  "  advance 
in  cut"  is  obtained  in  the  first  instance  by  the  alternate 
use  of  the  two  stern  spuds  and,  in  the  second,  by  "  walking" 
spuds.  The  third  function  is  accomplished  by  the  pump 
and  its  drive  with  the  appurtenant  suction  pipe,  floating 
discharge  pipe  or  pontoon  line,  and  the  shore  pipe-line. 
All  operations,  with  the  possible  exception  of  the  spud 
hoists,  are  controlled  from  the  pilot  house  or  lever-room. 
The  "runner"  or  "leverman"  faces  the  suction  end  and  is 


60  DREDGING  ENGINEERING 

guided  in  his  manipulation  of  the  dredge  by  pressure, 
vacuum,  steam  and  depth  gages  and  by  the  behavior  of  the 
cutter  engine. 

The  size  of  the  pump  varies  from  twelve  to  forty-eight 
inches,  but  the  twenty  inch  dredge  is  generally  conceded 
the  most  advantageous  for  all  around  work.  Twelve  and 
fifteen  inch  dredges  are  useful  for  dredging  in  confined 
areas  and  for  pumping  into  basins  of  small  capacity,  or  into 


FIG.  24. — The  Tampa — Atlas  Dredging  Co.,  New  York. 

filled  piers  or  behind  bulkheads.  The  larger  sizes  are  well 
adapted  to  the  removal  of  silt  and  fine  sand  and  to  the 
rehandling  of  material  previously  excavated  by  bucket 
dredges  and  dumped  from  scows  to  the  hydraulic  dredge 
to  be  pumped  ashore. 

Figure  23,  Page  59  is  a  photograph  of  the  thirty  inch 
dredge  PENNSYLVANIA,  of  the  American  Dredging  Com- 
pany, of  Philadelphia,  and  is  a  good  example  of  the  swing- 
ing ladder  type  with  walking  spuds*  This  dredge  has  been 
used  exclusively  for  rehandling  dredged  material,  and  has 
pumped  as  high  as  753,000  cubic  yards  of  mud  in  one 
month,  working  24  hours  per  day. 

Figure  24  Page  60  is  the  twenty-two  inch  hydraulic 
dredge  Tampa  of  the  Atlas  Dredging  Company  of  New 
York  City.  It  is  a  powerful  machine  of  the  swinging- 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 


61 


dredge  type  and  performed  yeoman  service  in  the  removal 
of  a  heavy  mixture  of  mud,  sand  and  some  gravel  incident 
to  the  construction  of  the  Hog  Island  Shipyard. 


FIG.  25a. — The  open  or  cage  cutter.     (Courtesy  of  Marion  Steam  Shovel  Co.) 


FIG.  25b — The  closed  or  round-nose  cutter.     30  inch,   light  type. 

of  Bucyrus  Co.) 


(Courtesy 


Cutter  Head  and  Ladder. — There  are  two  principal 
types  of  cutters,  the  open  or  cage  type  as  shown  in  Figure 
25a,  Page  61,  and  the  closed  or  round-nose  cutter  as 
illustrated  in  Figure  256.  The  former  consists  of  a  set 


62  DREDGING  ENGINEERING 

of  knives  or  blades,  straight  or  nearly  so,  mounted  on  a 
pair  of  annular  frames  of  different  diameters  so  that  the 
blades  converge  outboard.  The  blades  are  set  at  an  angle 
of  from  15  to  25  degrees  with  the  cutter  head  axis  or  shaft 
and  protrude  a  short  distance  beyond  the  end  frame.  The 
great  wear  caused  by  the  excessive  abrasion  makes  it 
advisable  that  the  blades  be  independent  of  the  frames  to 
facilitate  renewals.  In  the  closed  or  round-nose  cutter, 
the  blades  are  spiral  shaped  and  converge  to  a.  hub  at  the 
nose  or  outboard  end  into  which  the  end  of  the  shaft  is 
fitted.  In  both  types,  the  cutter  shaft  is  set  immediately 
above  the  suction  pipe,  the  end  of  which  must,  of  course, 
lie  within  the  surface  described  by  the  cutter  blades.  The 
large  diameter  of  the  cutter-head,  therefore,  will  be  some- 
what greater  than  twice  the  outside  diameter  of  the  suction 
pipe. 

The  relative  merit  of  the  two  cutters  will  vary  with  the 
character  of  the  material  and  even  more  perhaps  in  the 
judgement  of  dredging  men.  Mr.  Charles  Evan  Fowler, 
in  his  " Subaqueous  Foundations"  writes  "The  round-nose 
type  is  best  suited  to  soft  material,  but  usually  the  ordin- 
ary inward-delivery  cage  cutter  is  to  be  preferred  especially 
for  the  compact  and  harder  materials."  If  working  in 
clay,  a  cutter  with  a  relatively  large  number  of  blades 
closely  spaced  has  a  tendency  to  retain  the  clay  within 
itself,  finally  becoming  entirely  choked  with  the  sticky 
mass. 

To  exclude  stones  and  other  obstructions,  various 
straining  devices  are  used.  The  cutter  becomes  a  strainer 
when  fitted  with  a  series  of  bands  or  rings  at  the  blades  in 
planes  perpendicular  to  the  shaft,  resulting  in  a  cage-like 
structure,  admitting  only  such  objects  as  are  smaller  than 
the  trapezoidal  openings  between  the  blades  and  the 
transverse  rings.  The  material  is  excluded  from  the  rear 
or  large  end  of  the  cutter  either  by  a  transverse  screen  or 
by  a  solid  disc,  which  completes  the  closure  partially 
effected  by  suction  pipe  and  ladder.  Another  scheme 
involves  the  use  of  a  circular  transverse  screen  or  grating 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  63 

set  in  the  big  end  of  the  cutter,  perpendicular  to  the  shaft 
and  a  short  distance  in  front  of  the  end  of  the  suction  pipe. 
It  is  attached  to  the  cutter  and  rotates  with  it,  resulting 
in  the  continual  procession  of  the  strainer  openings  across 
the  mouth  of  the  suction.  It  is  apparent  that  any  long 
and  narrow  obstruction,  such  as  a  stick  of  wood  or  a  long 
stone,  passing  through  one  strainer  opening  and  protruding 
into  the  suction  pipe  would  be  subjected  to  considerable 


FIG.  26. — Ladder   and   cutter   machinery   for   20   inch   dredge.     Extra   heavy. 
(Courtesy  the  Bucyrus  Co.) 

shearing  strain  and  would  cause  high  stresses  in  the  strainer, 
cutter  and  shaft.  It  is  well,  therefore,  generously  to  pro- 
portion these  parts  that  they  may  be  capable  of  stalling 
the  cutter  engine  without  overloading  themselves. 

The  cutter  shaft  is  of  relatively  large  section  (about  eight 
inches  for  a  twenty-inch  dredge)  because  of  the  torsional 
stresses  set  up  in  it  by  the  maximum  moment  of  which 
the  cutter  engine,  through  its  gears,  is  capable.  It  is 
carried  on  pillow  block  journals  set  on  the  top  face  of  the 
ladder. 

The  ladder  may  be  a  heavy  steel  box  girder  enclosing 
the  suction  pipe;  or  a  pair  of  plate  girders  braced  together; 
or  a  pair  of  lattice  trusses.  The  last  mentioned  type  offers 
less  resistance  to  the  current  and  is  on  that  account  better 
adapted  for  use  in  swift  streams.  In  swinging  dredges, 


64  DREDGING  ENGINEERING 

the  ladder's  perpendicularity  to  the  bow  is  maintained  by 
guy  rods  to  the  forward  corners  of  the  hull,  or  by  spreading 
the  two  girders  forming  the  ladder  so  that  it  is  triangular 
in  plan,  or  by  a  combination  of  these  two  methods.  The 
stresses  in  the  ladder  are  due  to  the  pull  of  the  swinging- 
wires,  the  reaction  of  the  rotating  cutter,  horizontal  and 
vertical  hull  movement  when  the  cutter  head  is  in  the  mud 
and  finally  to  the  upward  pull  of  the  fall  from  the  boom  in 
raising  the  ladder  and  the  dead  weight  of  the  ladder  itself. 
The  ladder  is  set  in  a  well  or  recess  in  the  center  of  the  bow 
of  the  dredge.  In  the  case  of  the  swinging  ladder  dredge 
this  joint  must  permit  both  horizontal  and  vertical  motion 
of  the  ladder  with  respect  to  the  dredge,  but  in  the  swinging 
dredge  vertical  motion  only  is  required.  If  the  outboard 
end  of  the  latter  is  guyed  to  the  hull  corners,  the  points  of 
attachment  of  guy  rods  must  be  in  line  transversely  and 
vertically  with  the  center  line  of  the  hinge. 

The  maximum  depth  below  the  water  surface  to  which 
a  hydraulic  dredge  can  dig  is  determined  by  the  length  of 
the  ladder  and  the  hinge  construction.  Ladders  70  feet 
in  length  are  not  uncommon,  permitting  dredging  to  a  depth 
of  about  45  or  50  ft.  below  the  water. 

Feeding. — The  arrangement  of  spuds  depends  upon  the 
method  of  feeding.  The  case  of  the  swinging  dredge  will 
be  first  considered. 

It  has  but  two  spuds  set  in  the  same  transverse  plane 
at  or  near  the  stern  of  the  dredge,  about  10  or  12  feet  apart 
and,  if  the  discharge  pipe  is  centered  in  the  hull,  symmetri- 
cal with  respect  to  the  dredge  axis.  If  the  discharge  pipe 
leaves  the  hull  at  a  point  near  a  stern  corner,  the  spuds  are 
set  off  center  in  the  direction  of  that  same  corner,  in  order 
that  the  point  of  attachment  of  pontoon  line  to  dredge  shall 
be  as  close  as  possible  to  the  center  of  rotation  to  minimize 
the  effect  of  the  rotation  upon  the  alignment  of  the  pontoon 
line.  When  working,  one  spud  is  always  up,  clear  of  the 
bottom  and  the  other  down  in  the  mud,  forming  the  pivot 
about  which  the  dredge  rotates  or  swings.  This  radial 
motion  is  effected  by  two  swinging  wires,  extending  from 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 


65 


two  anchors,  one  on  each  side  of  the  bow,  to  the  winding 
drums  on  the  dredge.  The  pull  of  the  wires  may  b$  applied 
by  means  of  sheaves  to  the  outboard  end  of  the  ladder 
near  the  cutterhead,  to  the  bow  corners  of  the  hull  or  to  a 
kind  of  triangular  horizontal  apron  extending  forward  from 
the  bow.  The  first  method  has  the  advantage  of  keeping 


FIG.  27. — The  cutter-head  at  work.     (Courtesy  the  Bucyrus  Co.) 

the  swinging  wires  down  on  the  bottom  out  of  the  way  of 
passing  vessels  but  presents  also  the  disadvantage  of  the 
possibility  of  entanglement  of  the  slack  swinging  wire 
about  the  cutter  in  the  event  of  careless  operation.  When  it 
becomes  necessary  to  advance  the  dredge  in  order  again  to 
thrust  the  cutter  into  the  bank,  the  movement  is  accom- 
plished by  dropping  the  high  spud  and  raising  the  other 
when  the  dredge  has  swung  to  the  limit  of  the  cut.  Thus, 


66  DREDGING  ENGINEERING 

the  new  pivot  is  further  ahead  in  the  cut  and  the  center  of 
rotation  therefore,  advanced.  In  this  manner,  the  dredge 
moves  ahead,  by  the  alternate  use  of  the  two  spuds,  suc- 
cessive positions  of  which,  if  plotted  and  joined  by  straight 
lines,  would  form  a  series  of  saw  teeth. 

The  swinging-ladder  dredge  has  four  spuds,  two  forward 
and  two  aft,  all  of  which  are  in  the  mud  while  pumping. 
Thus  the  hull  is  held,  while  the  ladder  is  swung  by  means  of 
swinging  wires  from  cutter  head  to  winding  drums  after 
passing  through  guide  sheaves  at  the  bow  corners.  At 
least  three  of  the  four  spuds  are  set  in  slotted  wells,  per- 
mitting them  to  incline  forward  from  the  vertical  and  are 
called  "  walking  spuds. "  Two  wires  extend  from  the  two 
forward  spuds  at  or  near  their  toes  to  sheaves  on  the  stern 
corners  of  the  hull,  thence  to  drums  of  the  winding  engine. 
To  move  ahead,  these  wires  are  wound  on  the  drums, 
pulling  the  dredge  forward  and  inclining  the  spuds.  When 
desirable,  the  walking  spuds  can  be  raised  and  plumbed, 
one  at  a  time.  Such  dredges  usually  are  equipped  with 
a  stern  wire  for  hauling  astern.  Obviously,  the  swinging- 
ladder  dredge  makes  a  much  narrower  cut  than  the  swing- 
ing-dredge and  can,  therefore,  work  in  more  confined  spaces. 
Further  more,  the  absence  of  side  wires  is  often  an  advan- 
tage as  they  interfere  with  traffic  in  the  vicinity  of  the 
dredge.  On  the  other  hand,  the  total  time  lost  in  moving 
and  shifting  the  dredge  is  greater  for  the  swinging-ladder 
type  because  it  covers  less  area  with  one  set  up. 

It  is  well  to  remember  that  the  stress  in  one  swinging 
wire  is  nearly  always  greater  than  in  the  other,  due  to  the 
fact  that  the  rotation  and,  therefore,  the  reaction  of  the 
cutter  is  always  in  the  same  direction,  which  fact  at  times 
becomes  a  factor  in  locating  the  dredge  to  the  best 
advantage. 

Boom  "A"  Frame  and  Back  Legs. — Boom  and  "A" 
frame  are  set  well  forward,  the  former  at  a  fixed  angle  of 
from  30  to  45  degrees  with  the  horizontal,  the  latter  gener- 
ally vertical.  The  ladder  is  suspended  from  the  boom 
head  by  a  wire  fall,  which  is  purchase  rigged  to  the  ladder 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  67 

near  the  cutterhead  and  partially  bridled  to  relieve  the 
bending  stresses  in  the  ladder.  The  purchase  retards  and 
eases  the  vertical  motion  and  at  the  same  time  economizes 
in  engine  capacity.  The  boom  may  be  a  single  strut,  or, 
in  dredges  that  swing  as  a  unit,  it  may  be,  in  plan,  an  acute 
"A"  frame  to  prevent  lateral  movement.  The  boom  of 
the  swinging-ladder  dredge  necessarily  swings  with  the 
ladder,  its  heel,  therefore,  being  mounted  upon  the  same 
rotating  bearing  that  carries  the  ladder  trunnions.  The 
"A"  frame  varies  greatly  in  design  from  the  ordinary  tim- 
ber type  to  steel  frames  of  rectangular  and  even  polygonal 
outline.  The  stresses  in  boom,  "A"  frame  and  back-legs 
or  guys  may  be  obtained  graphically,  knowing  the  maxi- 
mum pull  of  the  ladder  hoisting  engines,  the  purchase 
ratio  and  the  angles  of  lead.  The  "A"  frame  and  back- 
guy  stresses  will  be  greater  in  the  swinging-ladder  type  than 
in  the  swinging  dredge,  becoming  maximum  for  the  same 
positions  of  the  boom  as  outlined  under  grapples. 

In  some  dredges,  built  for  use  in  localities  where  head 
room  is  a  consideration,  such  as  in  canals  with  limited 
clearance  under  bridges,  the  boom  angle  is  quite  flat  and 
the  topping-fall  almost  horizontal,  and  at  about  the  same 
elevation  as  the  roof  of  the  house,  which  is  but  one  story 
high.  A  house  boat  is  generally  an  indispensable  adjunct 
to  such  dredges  as  there  is  no  space  for  living  quarters  on 
the  dredge.  A  notable  example  of  this  is  afforded  by 
dredges  used  in  the  construction  and  maintenance  of  the 
New  York  State  Barge  Canal. 

The  Pipe  Line. — The  total  length  of  discharge  pipe 
line  comprises  three  distinct  parts;  First,  the  portion  on 
the  dredge  from  the  pump  to  the  point  of  leaving  the  hull ; 
second,  the  floating  or  pontoon  line;  and,  third,  the  shore 
line.  Since  the  first  constitutes  a  permanent  integral 
part  of  the  dredge  and  also  because  any  break  therein 
might  flood  the  hull  and  sink  the  ship,  it  is  made  of  heavy 
cast  iron  pipe  with  flanged  joints.  For  the  usual  central 
transverse  position  of  the  pump,  there  are  three  principal 
methods  of  discharge  pipe  arrangement  resulting  in  three 


68  DREDGING  ENGINEERING 

possible  locations  for  the  point  of  attachment  of  the 
pontoon  line.  Referring  to  Figure  28,  they  are  No.  1, 
a  direct  lead  from  the  pump  to  the  side  of  the  hull;  No.  2, 
a  stern  discharge  near  one  corner;  and  No.  3,  the  axial 
stern  location.  No.  1  discharge  is  limited  to  the  swinging- 
ladder  dredge,  since  in  the  swinging  dredge  type  it  is  essen- 
tial that  the  point  of  connection  of  pontoon  line  to  dredge 
be  close  to  the  spud  about  which  the  dredge  rotates. 
Location  No.  3  presents  the  objection  of  loss  of  head  due  to 
reverse  curvature.  For  general  use,  Discharge  No.  2 
appears  to  be  preferable.  Some  swinging-ladder  machines 
are  equipped  to  discharge  either  at  the  stern  or  side,  com- 


No.3 
No.2 


FIG.  28. — Discharge  pipe  arrangements  on  the  dredge. 

bining  Nos.  1  and  2,  or  1  and  3.  The  precaution  is  some- 
times taken  to  enclose  the  pump  in  a  watertight  trough  so 
that,  in  the  event  of  a  break  in  the  pipe,  the  hull  will  not 
be  flooded.  Location  No.  2  is  particularly  adapted  to  this 
safety  measure  in  that  the  side  of  the  hull  and  the  adjacent 
longitudinal  bulkhead  form  the  two  sides  of  such  a  trough 
and  easy  access  may  be  had  to  the  pipe  so  located  by  sub- 
stituting removable  hatches  for  the  deck  over  the  pipe. 
The  floating  line  from  the  hull  to  the  shore  pipe  consists 
of  a  series  of  pontoons,-  each  carrying  a  section  of  pipe  from 
20  to  50  feet  long,  so  coupled  to  each  other  as  to  provide 
the  necessary  flexibility  to  allow  the  dredge  to  swing  and 
advance  or,  in  other  words,  to  "wag  her  tail."  Both 
pontoon  and  shore  pipe  are  lap  or  spiral  riveted,  usually 
the  former,  the  sheets  varying  in  thickness  from  No.  10 
gauge  to  %  inch.  The  shore  pipe  is  built  in  sections  of 
such  length  as  to  preclude  a  weight  too  great  for  handling 
by  manual  labor  alone.  They  are  seldom  greater  than 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 


69 


70  DREDGING  ENGINEERING 

20  feet  long  and  frequently  only  16  feet.  The  pontoon  pipe, 
however,  is  built  in  long  lengths  to  reduce  the  number  of 
couplings  and  pontoons  and  is  usually  of  heavier  metal. 
The  sections  of  the  floating  line  are  coupled  either  by 
heavy,  rubber  sleeves  or  by  ball-joint  connections.  Be- 
cause of  the  short  life  and  expense  of  the  rubber  sleeves 
(they  are  seldom  less  than  10  ply)  the  ball  joint  method  is 
more  economical.  The  shore  pipe  is  laid  with  slip  joints, 
the  male  end  of  each  length  protruding  into  the  female 
end  of  the  next  in  the  direction  of  flow,  and  wired  together, 
a  pair  of  hooks  being  riveted  to  each  end  of  each  section 
for  this  purpose.  Leaks  in  such  unions  are  almost  the  rule 
rather  than  the  exception  and  are  plugged  by  wedges  made 
from  wooden  shingles  or  equally  convenient  shapes.  In 
tide  water,  the  varying  elevation  of  the  pontoon  line  with 
respect  to  the  shore  line  is  permitted  by  a  ball-joint  pipe. 

The  most  common  fittings  used  in  connection  with 
dredge  discharge  pipe  are  bends,  elbows,  "Y"s  and  gate 
valves.  The  necessity  often  arises  for  sharper  curvature  in 
the  pontoon  line  than  is  permitted  by  the  couplings  and  it 
is  met  by  the  insertion  of  one  or  two  elbows  in  the  line. 
The  shore  pipe  deviates  from  the  straight  line  in  circum- 
venting obstructions  and  in  continual  shifting  of  direction 
in  the  basin  to  control  the  elevation  of  the  fill.  Although 
the  slip  joints  allow  a  certain  amount  of  curvature,  bends 
of  15  degrees  and  up  are  often  essential  fittings.  In  order 
to  facilitate  the  addition  of  pipe  sections  in  the  basin  with- 
out stopping  the  pump,  it  is  common  practice  to  divide  the 
line  into  two  branches  by  means  of  a  "Y"  and  two  gate 
valves  at  a  point  on  the  fill  near  the  discharge,  with  the 
idea  that  one  leg  of  the  fork  can  be  lengthened  while  pump- 
ing continues  uninterruptedly  through  the  other.  For 
this  to  be  a  true  economic  advantage,  it  is  apparent  that 
the  loss  of  head  due  to  the  fittings  must  be  fully  offset  by 
the  prevention  of  lost  pumping  time. 

There  are  several  types  of  pontoon,  which  may  be  desig- 
nated as  the  "scow"  pontoon,  the  " catamaran"  pontoon 
and  the  "cask"  pontoon.  The  first  is  simply  a  single  box 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 


71 


of  wood  or  steel,  rectangular  in  plan  and  section.  The 
second  consists  of  two  long  narrow  floats,  either  steel  cylin- 
ders or  wood  boxes,  with  their  longitudinal  axis  perpendicu- 
lar to  the  pipe  mounted  upon  them  amidships.  By  struts 
from  one  to  the  other  of  each  pair,  they  are  maintained 
at  such  a  distance  apart  as  to  place  one  near  each  end  of  the 
pipe  section.  In  the  "cask"  pontoon,  the  pipe  rests 
upon  a  horizontal  frame  to  the  under  side  of  which  a  number 
of  strong  barrells  or  large  kegs  are  fastened,  providing 


FIG.  30. — The  pump  and  thrust  bearing.     (Courtesy  the  Bucyrus  Co.) 

the  requisite  buoyancy.  For  the  convenience  and  safety 
of  the  crew  and  pipe  gang,  it  is  desirable  to  provide  a  con- 
tinuous walkway  on  the  pontoons  parallelling  the  pipe. 
The  Pump. — Centrifugal  pumps,  as  used  for  dredging, 
have  the  following  features:  They  are  horizontal  pumps, 
i.e.,  the  shaft  is  horizontal.  They  are  high  pressure  pumps, 
i.e.,  the  head  for  which  they  are  designed  is  greater  than 
50  feet.  They  are  single  stage  pumps,  i.e.,  the  head  is 
generated  by  one  impeller  only.  They  are  single  inlet 
pumps,  i.e.,  the  water  enters  the  impeller  on  one  side  only, 
necessitating  the  interposition  of  a  marine  thrust  bearing 


72  DREDGING  ENGINEERING 

between  the  pump  and  its  drive.  They  are  backward- 
discharge  pumps,  i.e.,  the  impeller  tips  are  bent  backward 
and  the  angle  O,  Figure  31,  Page  72,  between  the  absolute 
velocity  of  exit  of  the  water,  FI,  and  the  peripheral  velocity 
of  the  impeller,  V2  is  greater  than  90  degrees.  This  type 
of  pump  is  the  most  common  even  in  water  pumps  and, 
because  the  head  decreases  as  the  quantity  increases,  it  is 
better  able  to  take  care  of  a  fluctuating  quantity  without 
overloading  its  driver.  It  is  this  feature  principally  which 
makes  it  the  most  desirable  type  for  dredging  purposes. 

There  are  no  discharge  vanes  in  dredge  pumps. 

The  impeller  is  of  the  enclosed  type.  The  open  type, 
due  to  rapid  wear,  loses  in  efficiency. 


FIG.  31. — Dredge  pump  impeller  or  runner. 

The  openings  between  blades  and  between  the  peri- 
phery of  the  impeller  and  the  shell  are  large  to  permit  the 
passage  of  large  objects  such  as  stones,  short  timbers, 
etc.  The  pump  must  be  designed  to  take  anything  that 
can  pass  through  the  cutterhead  and  into  the  suction. 
Chokes  in  the  pump  involve  loss  of  time  and  possibly 
serious  damage. 

Both  impeller  and  shell  are  heavily  proportioned  to 
withstand  the  abrasive  action  of  the  pumpings.  It  is  good 
practice  to  use  an  unlined  pump  of  very  heavy  proportions 
fitted  with  lined  side  discs. 

Shell  and  impeller  are  cast  iron  or  cast  steel. 

The  problem  is  to  design  a  pump  to  stand  the  unusually 
severe  wear  and  tear,  to  pass  fairly  large  bodies,  and  yet 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  73 

be  of  reasonable  efficiency.  It  is  apparent  that  the  effi- 
ciency of  a  dredge  pump  cannot  be  as  high  as  that  of  a 
water  pump,  and  seldom  exceeds  50  per  cent. 

The  total  head  to  be  overcome  by  the  pump  is  the  sum 
of  three  components:  first,  the  suction  head;  second,  the 
height  through  which  the  discharged  pumpings  are  raised, 
which  may  be  termed  the  elevation  head;  and  third,  the 
friction  head,  or  the  frictional  resistance  offered  by  the 
pipe  to  the  passage  of  the  pumpings.  The  pump,  therefore, 
must  create  a  partial  vacuum  in  the  suction  pipe  in  order 
to  give  to  the  entering  water  a  high  velocity,  sufficient  to 
draw  in  the  material  and  to  keep  it  moving  and  must  also 
produce  the  pressure  necessary  to  force  the  water  and  ma- 
terial through  the  discharge  pipe  against  the  resisting 
elevation  and  friction  heads.  Twenty-inch  pumps  have 
been  designed  capable  of  elevating  the  pumpings  to  a 
height  of  40  feet  through  3,000  feet  of  pipe  line. 

The  suction  head  is  the  amount  of  vacuum  in  the  suction 
pipe  and  is  read  on  the  vacuum  gage  in  the  lever  room  of  the 
dredge.  This  gage  is  usually  calibrated  in  inches  of  mercury, 
i.e.,  the  barometric  column.  Its  readings  may  be  converted 
into  feet  of  water  by  multiplying  by  the  constant  1.13 
which  figure  is  the  ratio  between  the  weight  of  a  column  of 
mercury  one  inch  high  and  a  column  of  water  one  foot  in 
height,  since  the  specific  gravity  of  mercury  is  13.56. 

The  elevation  head  is  the  vertical  height  from  the  center 
of  the  pump  to  the  highest  point  in  the  discharge  pipe  line. 
The  sum  of  the  friction  and  elevation  heads  is  obtained 
from  the  discharge  pressure  gage  in  the  lever-room.  The 
reading  is  in  pounds  of  pressure  per  square  inch  and  may 
be  converted  into  feet  of  head  by  multiplying  by  the  con- 
stant 2.304. .  As  this  gage  is  usually  some  distance  above 
the  center  of  the  pump,  the  head  represented  by  the  gage 
reading  must  be  increased  by  this  distance  in  order  to  get 
the  actual  discharge  head  on  the  pump.  "The  friction 
head  increases  as  the  velocity  of  discharge  increases  and 
decreases  as  the  pipe  diameter  increases.  It  is  directly 
proportional  to  the  length  of  pipe  line.  Curvature  in  the 


74  DREDGING  ENGINEERING 

line,  sleeve  and  ball  joints  in  the  pontoon  line  and  gate 
valves  develope  a  certain  amount  of  friction  head,  but, 
for  the  average  line  of  reasonable  straightness,  this  is  neg- 
lected and,  in  the  determination  of  the  value  of  the  friction 
head,  the  pipe  is  regarded  as  straight.  Hyraulic  dredging 
is  by  no  means  an  exact  science,  and  the  friction  head  in 
particular  varies  with  the  nature  of  the  material  dredged 
so  that  it  would  be  inconsistent  to  attempt  to  evaluate 
such  minor  head  losses  as  are  caused  by  pipe  fittings  and 
moderate  curvature. 

The  Table  on  Page  75  copied  from  the  Morris  Machine 
Works  Catalogue,  gives  full  data  as  to  friction  head, 
velocity  and  discharge  for  water  flowing  through  pipes. 
Since  the  frictional  resistance  is  greater  for  hydraulic 
dredgings  or  pumpings  than  for  water,  the  values  of  the 
head  in  this  table  must  be  increased  for  dredging  computa- 
tions by  an  amount  depending  upon  the  nature  of  the 
material  being  dredged.  By  multiplying  the  friction 
values  for  water  by  1.35,  very  good  results  are  obtained 
for  average  hydraulic  material.  The  increase  varies  from 
0.1  to  0.5. 

The  key  to  the  investigation  of  dredge  pump  performance 
is  the  relation  between  the  total  head  and  the  peripheral 
velocity  of  the  impeller  or  runner.  From  any  discussion 
of  the  theory  of  the  centrifugal  pump,  it  can  readily  be 
determined  that  the  peripheral  speed  is  directly  propor- 
tional to  the  square  root  of  the  total  head,  or  if 

P.  V.  =  the  peripheral  velocity  of  the  impeller 

H  =  the  total  head 
and  C  =  a  constant 
then  P.  V.  = 


The  value  of  the  constant,  C,  varies  somewhat  with  the 
capacity,  being  higher  for  higher  capacity;  also  with  the 
ratio  between  the  outside  diameter  of  the  runner  and  that 
of  the  throat  opening,  being  higher  for  smaller  diameter 
runners  with  the  same  throat  opening;  and  also  with 
the  angle  of  slope  of  the  vanes  at  the  periphery,  being 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE 


75 


CAPACITY  IN  GALLONS  PER  MINUTE  DISCHARGED  AT  VELOCITIES  IN  FEET 

PER  SECOND,  FROM  3  TO  15.     ALSO  FRICTION  HEAD  IN  FEET  PER  100 

FEET  LENGTH  OF  PIPE 


Diam. 
Pipe 

1-inch 

2-inch 

3-inch 

4-inch 

Diam. 
Pipe 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Veloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

3 

7.34 

4.08 

29.37 

2.04 

66.09 

1.36 

117.50 

1.02 

3 

4 

9.79 

6.83 

39.16 

3.41 

88.12 

2.27 

156.67 

1.71 

4 

5 

12.24 

10.2 

48.95 

5.12 

110.15 

3.41 

195.70 

2.56 

5 

6 

14.68 

14.3 

58.74 

7.16 

132.18 

4.78 

235.84 

3.58 

6 

7 

17.13 

19.0 

68.53 

9.54 

154.21 

6.36 

274.98 

4.77 

7 

8 

19.58 

24.5 

78.32 

12.2 

176.24 

8.16 

314.12 

6.12 

8 

SM 

20.80 

27.4 

83.23 

13.7 

187.25 

9.15 

333.75 

6.86 

SM 

9 

22.03 

30.5 

88.11 

15.2 

198.27 

10.1 

352.26 

7.64 

9 

9M 

23.25 

33.8 

93.00 

16.9 

209.24 

11.2 

371.90 

8.46 

9^ 

10 

24.48 

'37.3 

97.90 

18.6 

220.30 

12.4 

391.40 

9.33 

10 

10>£ 

25.70 

40.9 

102.80 

20.4 

231.31 

13.6 

411.05 

10.2 

10H 

11 

26.92 

44.7 

107.69 

22.3 

242.33 

14.9 

430.54 

11.1 

11 

11^ 

28.15 

48.7 

112.58 

24.3 

253.34 

16.2 

450.20 

12.1 

HM 

12 

29.37 

52.8 

117.48 

26.4 

264.36 

17.6 

470.68 

13.2 

12 

13 

31.82 

61.5 

127.27 

30.7 

286.39 

20.5 

509  .  82 

15.3 

13 

14 

34.27 

71.0 

137.06 

35.5 

308.42 

23.7 

548.96 

17.7 

14       4 

15 

36.72 

81.0 

146.85 

40.5 

330.45 

27.0 

587.10 

20.3 

15     * 

Diam. 
Pipe 

5-inch 

6-inch 

7-inch 

8-inch 

Diam. 
Pipe 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Veloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

3 

183.63 

.816 

264  .  24 

.68 

359.79 

.583 

470.04 

.510 

3 

4 

244.84 

1.36 

352.32 

1.13 

479.72 

.976 

626.72 

.854 

4 

5 

306.05 

2.05 

440.40 

1.70 

599.65 

1.46 

783  .  40 

1.28 

5 

6 

367.26 

2.86 

528.48 

2.38 

719.58 

2.05 

940.08 

1.79 

6 

7 

428.47 

3.81 

616.56 

3.18 

839.51 

2.72 

1096.7 

2.38 

7 

8 

489.68 

4.90 

705.64 

4.08 

959  .  44 

3.49 

1253.4 

3.06 

8 

520.61 

5.49 

749.01 

4.57 

1019.4 

3.92 

1331.5 

3.43 

9  2 

550.89 

6.11 

793.72 

5.09 

1079.4 

4.36 

1410.1 

3.82 

9  2 

581.25 

6.77 

837.08 

5.61 

1139.4 

4.83 

1488.0 

4.23 

10  * 

612.10 

7.46 

881  .  80 

6.21 

1199.3 

5.33 

1566.8 

4.66 

10  2 

642.43 

8.19 

925.20 

6.82 

1259.3 

5.84 

1645.8 

5.22 

11 

673.31 

8.95 

969.88 

7.45 

1319.2 

6.39 

1723.5 

5.59 

U/x 

11^ 

703  .  62 

9.74 

1013.3 

8.11 

1379.2 

6.95 

1801.5 

6.08 

12 

734.52 

10.5 

1057.9 

8.80 

1439.2 

7.54 

1880.2 

6.60 

12  2 

13 

795.73 

12.3 

1145.0 

10.2 

1559.1 

8.79 

2036.8 

7.00 

13 

14 

856  .  94 

14.2 

1233.1 

11.8 

1679.0 

10.1 

2193.5 

8.87 

14 

15 

918.15 

16.2 

1321.2 

13.5 

1799.0 

11.6 

2350.2 

10.1 

15 

D' 

T^lOTYl 

p'ipe' 

9-inch 

10-inch 

12-inch 

14-inch 

uiam. 
Pipe 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Teloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

3 

594.78 

.453 

734.40 

.408 

1057.5 

.347 

1439.0 

.291 

3 

4 

793  .  04 

.759 

979.20 

.683 

1410.0 

.581 

1919.7 

.488 

4 

5 

991.30 

1.13 

1224.0 

1.02 

1762.6 

.871 

2399  .  4 

.731 

5 

6 

1189.5 

1.59 

1468.8 

1.43 

2115.1 

1.21 

2878.0 

1.02 

6 

7 

1388.8 

2.12 

1713.6 

1.90 

2467.6 

1.62 

3358.7 

1.36 

7 

8 

1586.0 

2.72 

1958.4 

2.45 

2820.1 

2.08 

3838.4 

1.75 

8 

SM 

1685.0 

3.05 

2080.8 

2.74 

2996.3 

2.33 

4078.3 

1.96 

8H 

9 

1784.3 

3.40 

2203.2 

3.05 

3172.7 

2.60 

4318.1 

2.18 

9 

9K 

1883.5 

3.76 

2325.6 

3.38 

3348.9 

2.88 

4558.0 

2.42 

9K 

10 

1982.6 

4.14 

2448.0 

3.73 

3525.2 

3.17 

4798.0 

2.66 

10 

10M 

2082.7 

4.55 

2570.8 

4.09 

3701.4 

3.48 

5037.7 

2.92 

IOH 

11 

2181.9 

4.97 

2692.8 

4.47 

3877.7 

3.80 

5277.5 

3.19 

11 

UK 

2280  0 

5.41 

2815.2 

4.87 

4053  .  8 

4.14 

5517.4 

3.48 

HH 

12 

2279.1 

5.87 

2937.6 

5.28 

4230.2 

4.49 

5757.2 

3.77 

12 

13 

2577.4 

6.84 

3182.4 

6.15 

4582.8 

5.23 

6237.8 

4.40 

13 

14 

2776.6 

7.88 

3427.2 

7.10 

4935.4  . 

6.03 

6717.5 

5.06 

14 

15 

2974  .  9 

9.00 

3672  .  0 

8.10 

5287.8 

6.89 

7107.2 

5.79 

15 

76 


DREDGING  ENGINEERING 


CAPACITY  IN  GALLONS  PER  MINUTE  DISCHARGED — Continued 


Diam. 
Pipe 

15-inch 

18-inch 

20-inch 

22-inch 

PT^' 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Veloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

it 

10(32.2 

.272 

2379.7 

.227 

2937 

.204 

3554.1 

.185 

3 

4 

2208  .  0 

.455 

3172.6 

.379 

3916 

.342 

4739.8 

.310 

4 

5 

2754.7 

.682 

3965.5 

.569 

4896 

.512 

5924.5 

.465 

5 

(5 

3304  .  4 

.  955 

4758.4 

.795 

5875 

.717 

7108.2 

.651 

6 

7 

3855.2 

1.27 

5552.3 

1.06 

6854 

.954 

8293  .  9 

.866 

7 

8 

4406.9 

1.63 

6345.2 

1.36 

7833 

1.22 

9478.6 

1.11 

8j 

4688.1 

1.82 

6741.9 

1.52 

8323.6 

1.37 

10071 

.25 

9   2 

4957.7 

2.04 

7138.1 

1.70 

8812 

1.53 

10663 

.39 

9 

5232.1 

2.25 

7534.8 

1.88 

9302.6 

1.69 

11255 

.54 

9H 

10 

5508.4 

2.50 

7931.0 

2.07 

9792 

1.87 

11848 

.69 

10 

10K 

5783.4 

2.73 

8328.8 

2.27 

10281 

2.05 

12440 

.86 

10  K 

11 

(5058.2 

2.98 

8724  .  9 

2.48 

10771 

2.24 

13033 

2.03 

11 

11  K 

6334.6 

8  .  25 

9121.7 

2.70 

11258 

2.43 

13625 

2.21 

UK 

12 

6609.9 

3  .  52 

9517.8 

2.93 

11750 

2.64 

14217 

2.40 

12 

13 

7160.6 

4.10 

10310 

3.42 

12729 

3.08 

15402 

2.79 

13 

14 

7711.4 

4.78 

11104 

3.93 

13708 

3.55 

16587 

3.22 

14 

'15 

8262 

5  .  40 

11897 

4  .  50 

14(588 

4.05 

17772 

3.68 

15 

Pipe 

24-inch 

26-inch 

28-inch 

30-inch 

Diam. 
Pipe 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Veloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

3 

4230.3 

.170 

4964  .  2 

.157 

5757.2 

.146 

6609 

.136 

3 

4 

5640.0 

.284 

6619.0 

.262 

7676.2 

.244 

8812 

.227 

4 

5 

7050.8 

.426 

8274.7 

.394 

9596.3 

.366 

11015 

.341 

5 

6 

8460  .  (5 

.597 

9929.5 

.550 

11514 

.512 

13218 

.478 

6 

.      7 

9870.3 

.794 

11583 

.753 

13434 

.681 

15421 

.636 

7 

8 

11280 

1.01 

13238 

.940 

15353 

.875 

17624 

.816 

8 

11985 

.14 

14066 

1.05 

16316 

.980 

18725 

.915 

9 

12690 

.27 

14893 

1.17 

17273 

.09 

19827 

1.01 

9 

0^ 

13395 

.40 

15721 

1.30 

18231 

21 

20928 

1.12 

9K 

10 

14100 

.55 

16548 

1.43 

19192 

.33 

22030 

1.24 

10 

14805 

.70 

17375 

1.57 

20150 

.46 

23131 

1.36 

10M 

11 

15510 

.86 

18202 

1.72 

21111 

.60 

24233 

1.49 

" 

16215 

2  .  03 

19029 

1.87 

22069 

1.74 

25338 

1.62 

12 

16920 

2.20 

19857 

2.03 

23030 

1.89 

26436 

1.76 

12 

13 

18380 

2.56 

21511 

2.36 

24950 

2.20 

28639 

2.05 

13 

14 

19740 

2.95 

23166 

2.73 

26869 

2.53 

30842 

2.37 

14 

15 

21150 

8  .  37 

24824 

3.11 

28788 

2.89 

33045 

2.70 

15 

Diam. 
Pipe 

32-inch 

36-inch 

42-inch 

48-inch 

Diam. 
Pipe 

Veloc- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Capac- 

Fric- 

Veloc- 

ity 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

tion 

ity 

3 

7519.7 

.127 

9518 

.118 

12954 

.097 

16921 

.085 

3 

4 

10026 

.213 

12690 

.189 

17272 

.163 

22561 

.  143 

4 

5 

12532 

.  320 

15863 

.284 

21590 

.244 

28201 

.213 

5 

6 

15039 

.447 

19036 

.397 

25908 

.341 

33841 

.298 

6 

7 

17546 

.591 

22208 

.528 

30226 

.454 

39482 

.397 

7 

8 

20052 

.764 

25381 

.679 

34544 

.583 

55122 

.510 

8 

8K 

21306 

.857 

26967 

.760 

36704 

.653 

47942 

.571 

8K 

9 

22559 

.954 

28554 

.847 

38863 

.728 

50762 

.636 

9 

9M 

23812 

1.06 

30140 

.938 

41022 

.806 

f,.'5o82 

.694 

9K 

10 

25065 

1.16 

31726 

1.03 

43181 

.888 

56403 

.778 

10 

10M 

26319 

1.28 

33313 

1.13 

45340 

.975 

59223 

.851 

10K 

11 

27572 

1.40 

34899 

1.16 

47499 

1.06 

62043 

.930 

11 

UK 

28825 

1.52 

36485 

1.35 

49658 

1.16 

64863 

1.00 

\\H 

12 

30379 

1  .  65 

38072 

1.46 

51817 

1.26 

67683 

1.10 

12 

13 

32585 

1.92 

41244 

1.70 

56135 

1.46 

73324 

1.28 

13 

14 

35092 

2.21 

44417 

1.97 

60453 

1.69 

78964 

1.48 

14 

15 

3759* 

2.53 

47590 

2.24 

64771 

1.93 

84604 

1.69 

15 

HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  71 

smaller  for  larger  angles.  The  determination  of  the 
constant  is  purely  a  matter  of  judgment  with  the  above 
for  a  guide,  but  for  average  practice  a  value  of  435  may 
be  used  with  very  good  results.  Therefore,  when  H  is  in 
feet  and  P.  V.  in  feet  per  minute. 

P.V.  =435  VH  (1.) 

From  this  important  equation,  the  number  of  revolutions 
per  minute  necessary  for  an  impeller  to  turn  that  it  may 


FIG.  32. — 20  inch  pump  driven  by  1000  h.p.' triple  expansion  engine.     (Courtesy 
of  Morris  Machine  Works.) 

develop  a  given  head  can  be  determined;  or,  having  the 
R.  P.  M.  and  total  head,  the  proper  idiameter  of  impeller 
can  be  selected;  or,  knowing  the  R.  P.  M.  and  the  impeller 
diameter  of  any  machine,  the  head  of  which  it  is  capable 
can  be  found. 

In  this  connection,  it  is  well  to  remember  that  the  diam- 
eter of  the  impeller,  for  efficient  operation,  should  never 
be  less  than  2.3  times  the  size  of  the  pump;  e.g.,  a  20-inch 
pump  requires  a  runner  at  least  46  inches  in  diameter. 


78 


DREDGING  ENGINEERING 


Power.  —  The  theoretical  horse  power  developed  by  the 
pump  is  equal  to  the  continued  product  of  the  discharge 
in  gallons  per  minute  by  the  weight  of  one  gallon  of  water 
in  pounds  by  the  total  head  in  feet  divided  by  33,000  foot 
pounds  per  minute,  or  if 

Q  =  the  discharge  in  gallons  per  minute 
and  H  =  the  total  head  in  feet,  then 

rnu  *•        1    TT     -D  Q    X    8.33     X'H 

Theoretical  H.  P.  . 


or 


T,        w  P        Q  X  H 
Theo.  H.  P.  =  -gg^- 


(2.) 


FIG.  33. — 20  inch  pump  direct  connected  to  600  h.p.  motor,  one  of  four  built 

for  Panama. 

The  actual  horse  power  required  to  drive  the  pump  is 
about  twice  the  above  theoretical  value  since  the  efficiency 
of  a  dredge  pump  is  generally  about  50  per  cent,  or  a  trifle 
less. 

For  maximum  economy  of  power,  the  velocity  through 
the  pipe  should  be  no  more  than  just  enough  to  carry  the 
material  along  because  higher  velocity  means  increased 
friction  head  and  increased  quantity,  with  both  of  which 
the  horse  power  varies  directly.  For  heavy  material, 
such  as  coarse  sand  and  gravel,  however,  the  velocity 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  79 

must  be  higher  than  for  silt,  mud  and  fine  sand  to  prevent 
the  settlement  of  the  material  in  the  pipe  and  the  conse- 
quent choking. 

Long  pipe  lines  require  a  large  impeller  to  give  the  high 
peripheral  velocity  necessary  to  overcome  the  great  head. 
However,  should  the  same  impeller  be  used  on  short  lines, 
the  engine  speed  would  be  so  retarded  as  to  prevent  the 
engine  from  delivering  its  maximum  power.  The  smaller 
impeller  in  the  short  line  allows  greater  engine  speed, 
maximum  power  and,  therefore,  greater  quantity  of 
material  pumped.  It  is  obviously  advantageous,  there- 
fore, that  a  dredge  intended  for  miscellaneous  work  have 
several  impellers  of  different  diameters,  increasing  the  size 
with  lengthening  pipe  line. 

Method  of  Design. — Let  us  assume  that  it  is  desired  to 
design  a  hydraulic  dredge  capable  of  raising  average 
hydraulic  material  15  feet  above  the  pump  through 
4000  feet  of  pipe  line  and  at  the  same  time  be  suitable  for 
general  all  round  work  involvingalso  short  lines  and  low 
heads. 

It  is  generally  accepted  that  a  20  or  22  inch  pump  is  the 
most  advantageous  for  these  conditions.  We  shall  select 
the  20  inch.  The  next  step  is  to  choose  the  desired  velocity 
of  flow  through  the  4000  feet  of  line.  Ordinarily  the 
velocity  should  not  be  less  than  12  feet  per  second,  but  as 
this  length  of  pipe  is  the  ultimate  condition  and  as  the 
assumption  of  a  high  velocity  here  would  result  in  an 
excessively  high  velocity  for  short  lines  even  with  a  smaller 
impeller  in  the  pump,  we  will  fix  10  ft.  per  second  as  the 
proper  figure  for  4000  feet  of  line  and  an  elevation  head 
of  15  feet.  Entering  the  table,  Page  76,  we  find  that  for 
10  feet  velocity  in  a  20  inch  pipe,  the  friction  head  per 
100  feet  of  pipe  is  1.87  feet.  But  this  value  is  for  water 
and  must  be  multiplied  by  1.35  to  obtain  the  head  for 
dredgings,  which  is  2.52  feet,  and  the  friction  head  for 
4000  feet  of  pipe  is  40  X  2.52  or  100.8  feet.  The  pump 
should  be  able  to  hold  a  vacuum  of  about  12  inches  of 
mercury. 


80  DREDGING  ENGINEERING      ; 

Therefore, 

Elevation  head . =  15.0ft. 

Friction  head =  100.8ft. 

Suction  head  12  X  1.13  .  =  13.6ft. 


Total  Head  ......    .    .         129.4ft. 

or  say  130  ft. 

The  peripheral  velocity  of  impeller  necessary  to  develop 
this  head  is  found  by  formula  (1)  Page  77. 

P.  V.  =  435  V130  =  4970.  feet  per  minute 
bearing  in  mind  that  the  constant  435  involves  the  use  of  the 
usual  type  of  impeller  as  previously  described.  The  engine 
speed  for  dredges  of  this  size  on  long  lines  appears  to  be 
generally  from  about  200  to  225  revolutions  per  minute. 
Let  us  assume  for  our  problem  a  value  of  220  R.P.M. 

4970 
Then  the  required  diameter  of  impeller  is  „  ^ 

or  7.19  feet  or  86  inches. 

The  water  horse  power  for  this  condition  is  from  formula 

(2)  Page  78,  -  ^7:  -  =  322,  and,  for  a  pump  efficiency 


of  50  per  cent  the  engine  brake  horse  power  would  be  twice 
322  or  about  650. 

Now  let  us  investigate  the  same  pump  under  a  low  head. 
Assume  a  short  line,  say  1200  feet  and  a  low  lift,  10  feet. 
Using  the  same  impeller  and  engine  speed  as  above,  the 
developed  head  is  again  130  feet,  of  which  13.6  feet  is 
suction,  10  ft.  elevation,  and  the  balance,  106  ft.  friction 
head.  For  a  1200  ft.  line,  the  friction  loss  per  100  ft. 
of  pipe  is  8.83  ft.  corresponding  to  a  velocity  of  about 
19.5  ft.  per  second,  which  is  excessive.  The  remedy  is 
either  reduced  engine  speed  or  a  smaller  impeller  or  both. 
Suppose  an  impeller  78  in.  in  diameter  be  substituted  for 
the  86  in.  and  driven  at  the  same  speed  and  under  the 
same  low  head  conditions  as  above. 

p.V.  =  6.5  X  3.142  X  220  =  4500 

/4500V 
and  the  developed  head  •M^')  =  107  ft-     Tne  friction 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  81 

head  =  107  -  24  =  83  ft.  or  6.91  ft.  per  100.     For  water, 

6  91 
the  equivalent  head  =  ^Vc  =  5.12,  and   the  velocity  is 

1  .oO 

again  excessive,  approximating  17  ft.  per  second,  requiring 

,    ,,  ,  .     ,      ,    16,500  X  107  X  2 

a  shaft  horse  power  of  about  3960  =  900. 

The  high  velocity  may  be  reduced  by  increasing  the  size 
of  the  discharge  pipe  or  by  reducing  the  K.P.M.  Still 
using  the  78  inch  impeller,  but  cutting  the  engine  speed  to 

(fi  *»  V  3  1  42  V  200\  2 
' 


or  88  ft.;  the  friction  head  is  88  -  24  =  64  ft.  or  5.33  ft. 
per  100  ft.  of  the  1200  ft.  line.  Dividing  by  1.35  we  enter 
the  table  with  a  value  3.95  and  find  that  the  corresponding 
velocity  is  almost  15  ft.  per  second  and  the  discharge  about 
14,500  gallons  per  minute.  The  engine  horse  power  required 
.  14,600  X  88  X  2 
is  -  °r 


The  foregoing  leads  us  to  the  following  conclusions:  — 
that,  for  the  problem  stated,  the  solution  appears  to  be  a 
20  in.  pump  with  20  in.  suction  and  discharge,  having  at 
least  two  sizes  of  impeller,  about  78  in.  for  short  and  86 
in.  for  long  lines,  driven  by  a  triple  expansion  steam  engine 
of  about  800  horse  power,  turning  over  from  200  to  225 
R.P.M.  Although  this  is  a  greater  horse  power  than  theo- 
retically required  according  to  the  above  figures,  it  is  rec- 
ommended because  of  the  necessarily  uncertain  nature  of 
the  factors  involved  and  the  desire  for  reserve  power  to 
take  care  of  severe  pumping  conditions. 

The  Machinery.  —  The  pump  drive  must  be  capable  of 
variable  speed  and  of  running  at  different  speeds  for  long 
periods  of  time  because  the  load  on  the  pump  is  variable 
due  to  the  fluctuating  suction  head,  the  nature  of  the  ma- 
terial and  the  varying  length  of  pipe  line.  A  steam  engine 
meets  this  condition  fully  and  is  admirably  adapted  to  the 
purpose.  If  the  pump  is  driven  electrically  the  motor 
must  be  designed  for  this  varying  load.  A  synchronous 
motor  is,  therefore  unsatisfactory.  Although  electrically 


82 


DREDGING  ENGINEERING 


driven  machines  have  been  successfully  operated,  the  usual 
drive  is  a  triple  expansion  vertical  condensing  engine,  di- 
rectly connected  to  the  pump  and  called  the  main  engine. 
The  20  in.  dredge  NEW  JERSEY,  shown  on  Page  69,  has 
cylinders  18  and  24  and  40  inches  in  diameter  with  a  com- 
mon stroke  of  20  inches,  200  R.P.M.,  and  developes  750  h.p. 
The  22  inch  machine  " TAMPA"  pictured  on  Page  60,  has 
cylinders  14  and  21^  and  36  inches,  18  inch  stroke,  225 
R.P.M.  and  800  horse  power.  The  boilers  are  variously 
Scotch,  Heine  or  Almy  water  tube,  and  B.  &  W.  with  a 
working  pressure  of  from  175  to  225  pounds. 


500 


1000 


3000 


3500 


4000 


1500  2000  2500 

Gallons  per  Minute 

FIG.  34. — Characteristic    curves — dredge    pump.     Constant    speed.     (Courtesy 
the  Morris  Machine  Works.) 

Between  the  pump  and  the  main  engine,  a  thrust  bearing 
on  the  shaft  is  required  because  of  the  axial  pull  of  the 
impeller.  The  shaft  bearing  at  the  impeller  is  kept  clean 
by  a  water  service. 

The  cutter  engine  will  be  about  12  X  12  double  cylinder 
for  a  20  in.  machine  and  the  winding  engine  about  8>i  X 
12  double  cylinder,  driving  at  least  5  drums  for  swinging 
wires,  spuds  and  ladder  hoist,  or  more  if  the  dredge  be  of 
the  swinging  ladder  type  with  walking  spuds  and  stern 
wire. 

In  addition  to  the  above,  there  are  the  condenser  and 
centrifugal  circulating  pump,  the  air  pump,  the  generator 
set  for  electric  lighting  and  the  other  auxiliaries. 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  83 

Operation. — The  leverman  has  full  control  of  the  mani- 
pulation of  the  dredge.  The  lever-room  is  located  well 
forward  and  at  such  an  elevation  that  the  leverman  can 
keep  a  watchful  eye  on  the  swinging  of  the  machine  or 
ladder,  the  ranges  to  which  he  is  working,  a  tide  gage, 
the  behavior  of  the  cutter  engine  and  the  condition  of  the 
floating  pipe  line.  In  the  lever-room,  are  the  vacuum, 
discharge-pressure  and  steam-pressure  gauges,  which  guide 
the  operator  in  the  normal  operation  of  the  pump  and  feed. 
The  vacuum  in  the  suction  pipe  is  greater  when  pumping 
solids  than  when  water  only  is  passing  through,  becoming 
maximum  when  the  suction  is  choked.  The  discharge 
pressure  falls  off  for  chokes  in  the  suction  and  rises  for 
obstructions  in  the  discharge  pipe.  The  leverman  learns 
to  keep  the  gauge  readings  at  that  point  at  which  the  pump 
will  carry  the  maxmum  amount  of  material  without  choking. 
Both  vacuum  and  pressure  readings  acquaint  the  operator 
of  restricted  suction,  the  former  by  rising  and  the  latter 
by  falling,  but  the  vacuum  gage  is  more  sensitive  than  the 
pressure,  responding  more  quickly  to  the  abnormal  condi- 
tion. Through  gratings  in  the  floor  of  the  lever-room,  the 
operator  keeps  himself  informed  as  to  the  amount  of  swing- 
ing wire  left  on  the  drums  in  the  room  below  him.  The 
depth  of  the  cutterhead  below  the  water  surface  is  indicated 
by  a  sliding  weight  on  the  boom  or  by  a  dial  in  the  lever- 
room,  operated  through  reduction  tackle. 

Under  the  most  favorable  conditions,  in  mud  and  silt, 
hydraulic  dredges  may  reach  a  maximum  solid  output  of 
25  to  30  per  cent  of  the  pumpings,  but  in  average  digging, 
the  percentage  will  be  more  nearly  10  to  possibly  15  and 
in  heavy  material  with  long  lines  it  will  fall  as  low  as  5. 

Booster  or  Relay  Pumps. —  When  a  discharge  pipe  line 
becomes  so  long  as  to  reduce  the  output  of  the  dredge  below 
the  economic  minimum,  material  assistance  may  be  ren- 
dered the  dredge  pump  by  placing  in  the  shore  line  a  sec- 
ond pump  called  a  booster  or  relay  pump.  The  dredge 
discharges  directly  into  the  suction  of  the  booster,  which 
then  forces  the  pumpings  through  the  remaining  length  of 


84  DREDGING  ENGINEERING 

line  between  itself  and  the  fill.  Thus  the  dredge  pump  is 
relieved  of  the  length  of  line  beyond  the  booster  and  is 
enabled  to  accelerate  the  pipe  velocity  and  to  increase  the 
quantity  of  discharge.  It  is  apparent  that,  for  maximum 
efficiency  of  the  combination,  the  booster  must  discharge  an 
amount  equal  to  that  delivered  to  it  by  the  dredge.  If  the 
booster  receives  more  than  it  can  handle,  the  output  of  the 
dredge,  is  decreased  by  retarded  velocity,  although  it  may 
be  greater  than  in  the  long  line  without  the  booster.  If 
the  booster  is  capable  of  a  greater  discharge  than  that 
delivered  to  it,  it  will  draw  air  into  its  suction  and  pump 
the  mixture.  It  is  difficult  to  make  the  ordinary  dredge 
shore  pipe  air  tight.  If  the  dredge  discharge  is  sufficiently 
less  than  that  of  the  booster,  the  load  on  the  latter  will 


Line  to  Basin 


Valve  B 

Booster  Pump 

FIG.  35. — Typical  booster  installation. 

be  intermittent,  i.e.,  the  booster  receives  a  column  of 
pumpings  and  disposes  of  it  so  quickly  that  the  continuity 
of  the  supply  is  broken,  leaving  a  void  between  supply 
columns  during  which  the  impeller  rumbles  around  in  a 
mixture  of  air  and  water  before  receiving  the  next  column, 
which  is  as  quickly  discharged.  Consequently  the  dis- 
charge at  the  end  of  the  pipe  on  the  fill  reveals  a  very  appre- 
ciable pulsation.  It  is  apparent,  therefore,  that  there  is 
one  best  place  in  the  line  at  which  to  install  a  given  booster, 
and  the  usual  problem  will  be  the  determination  of  that 
point  rather  than  to  design  or  select  a  booster  for  a  particular 
point,  since  neither  are  dredging  operations  so  permanent 
nor  boosters  so  plentiful  as  to  afford  frequent  opportunities 
for  the  latter. 

The  solution  of  the  problem  again  involves  the  question 
of  peripheral  speed  of  impellers.  Knowing  the  diameter 
and  R.P.M.  of  the  dredge  and  booster  impellers,  select  a 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  85 

point  in  the  line  for  trial.  Compute  the  pipe  velocity 
created  by  the  dredge  for  the  length  of  line  from  dredge  to 
booster.  Compute  the  pipe  velocity  created  by  the  booster 
for  the  length  of  line  beyond  it  not  forgetting  that  this  length 
will  vary  usually  by  the  periodical  addition,  of  extra  pipe 
sections.  Reasonable  variations  in  pipe  length  can  be 
taken  care  of  by  varying  the  speed  of  the  drives  on 
either  dredge  or  relay,  or  both.  For  large  differences,  a 
second  impeller  for  the  booster  may  be  necessary.  By 
a  little  manipulation  the  point  in  the  line  at  which  the  relay 
must  be  located  to  insure  equal  pipe  velocities  of  dredge 
and  booster  can  be  determined. 

From  the  above,  it  is  obvious  that  the  booster  drive  must 
be  capable  of  variable  speed.  If  electrically  driven,  as  most 
of  them  are,  to  facilitate  transportation,  installation  and  op- 
eration, the  motor  should  be  designed  to  run  for  long  periods 
at  various  speeds  without  overheating.  Synchronous 
motors  are,  therefore,  wholly  unsatisfactory  for  this  purpose. 

The  pump  is  directly  connected  to  the  motor  with  a 
thrust  bearing  between  the  two.  The  latter  need  not  be 
so  heavy  as  that  on  the  dredge  since  the  water  enters  the 
pump  normally  without  vacuum.  It  is  desirable  to  have 
a  flexible  shaft  coupling  near  the  motor  to  provide  for 
changes  in  alignment.  The  behavior  of  the  pump  should 
be  carefully  watched  by  means  of  vacuum  and  pressure 
gages  on  the  suction  and  a  pressure  gage  on  the  discharge. 
The  vacuum  gage  should  read  zero  for  normal  behavior. 
The  load  on  the  motor  must  also  be  noted  by  gauge.  It 
is  almost  -essential  that  there  be  a  by-pass  line  around  the 
booster  so  that  it  may  be  cut  out  of  the  line  for  repairs 
or  other  causes  without  interruption  to  the  dredge  pump, 
and  that  it  may  be  given  the  load  gradually,  in  starting, 
to  prevent  an  overload  on  the  motor.  The  by-pass  is 
effected  by  two  "  Y"  branches  in  the  line,  one  beyond  each 
end  of  the  booster.  The  "Y"  branch  between  the  dredge 
and  booster  must  have  a  gate  or  valve  in  each  leg  but  that 
beyond  need  have  a  gate  in  the  booster  leg  only.  A 
typical  installation  is  shown  in  Figure  35,  page  84. 


86  DREDGING  ENGINEERING 

To  throw  the  booster  into  the  line,  the  procedure  is  as 
follows :  Start  the  motor  with  valves  B  and  C  closed ;  have 
the  dredge  pump  water  only;  leaving  valve  A  open,  open 
C  partly  then  B  about  the  same  distance;  open  B  wide; 
then  C  all  the  way;  close  A]  have  the  dredge  pump  mud. 
The  reasons  are  obvious.  The  minimum  load  on  the  motor 
occurs  with  the  valve  C  closed,  giving  maximum  head, 
zero  discharge  and  therefore  a  minimum  power  requirement 
which  is  the  desired  starting  condition.  There  is  consid- 
erable danger,  however,  of  blowing  the  line  apart  at  C 
if  pumping  against  that  valve  closed.  Hence  it  is  partially 
opened. 

The  installation  should  be  housed,  and  it  is  essential 
that  telephone  communication  or  some  system  of  signalling 
be  established  between  dredge  and  booster  so  that  the 
orders  to  stop,  start,  pump  water  and  pump  mud  may  be 
quickly  conveyed. 

FORWARD  FEEDING  OR  MISSISSIPPI  RIVER  TYPE 

General  Description. — The  maintenance  of  low-water 
navigation  in  the  shallow  waters  of  alluvial  rivers  through 
the  persistent  and  shifting  sand-bar  formations  is  a  prob- 
lem requiring  special  treatment.  In  the  Mississippi  River, 
these  bars,  during  seasons  of  high  water,  assume  extensive 
proportions,  but  as  the  river  falls,  they  are  cut  out  by  the 
current  erosion,  and  it  is  to  assist  and  hasten  this  natural 
deepening  tendency  that  the  dredges  are  designed.  To 
complicate  matters,  the  usual  dredging  season,  i.e.,  that 
portion  of  the  year  during  which  the  state  of  low  water 
exists;  is  only  four  months  in  length — from  August  15 
to  December  15.  (The  river  has  a  range  in  stage  of  up- 
wards of  50  feet  in  places.)  The  amount  of  dredging  to  be 
done  in  any  one  season  cannot  be  predicted,  because  of  the 
impossibility  of  forecasting  the  stage  or  the  rapidity  of 
fluctuation.  The  only  solution,  therefore,  appears  to  be 
simply  that  the  dredges  be  available  during  the  low-water 
season,  to  do  whatever  dredging  may  be  required. 


HYDRAULIC  DREDGES  OF  THE  RIVER  TYPE  87 

After  repeated  failures  to  make  and  maintain  the  depth 
necessary  for  low-stage  navigation  by  means  of  current 
deflectors,  water  jets,  stirring  and  scraping  machines  and 
other  devices,  the  Mississippi  River  Commission,  in  1892, 
built  the  30  inch  hydraulic  dredge,  ALPHA,  for  experimental 
purposes.  The  ALPHA  proved  so  successful  that  eight 
additional  machines  were  built  by  the  Commission  within 
the  next  decade.  Various  improvements  and  changes 
were  made  in  the  successive  dredges,  as  experience  dictated, 
until  the  type  of  to-day  was  developed  and  the  Mississippi 
problem  solved. 

The  typical  Mississippi  River  Dredge  differs  from  the 
radial  feeding  machine  with  spud  anchorage,  principally 
in: 

1 .  Greater  capacity  and  lighter  draft,  both  of  which  features 
are  necessitated  by  local  conditions. 

2.  The  Method  of  Feeding. — While  some  units   of  the 
Mississippi  River  plant  are  self  propelling,  being  equipped 
with  side  paddle-wheels,  many  are  pulled  forward  over  the 
bar  to  be  dredged  by  two  cables  extending  from  hauling 
engines  on  the  forward  deck  to  mooring  piles  driven  in 
advance  of  the  dredge.     For  holding  the  dredge  in  position 
while  running  lines  or  shifting  the  mooring  piles,  a  single 
spud  is  set  well  forward. 

3.  The  Use  of  Water  Jet  Agitators. — On  the  Mississippi 
the  mechanical  cutter  has  been  largely  replaced  by  the 
water-jet  agitator.     The  suction  head,  which  is  at  the  bow 
of  the  dredge — as  is  the  pump  also — is  flattened  down  to  a 
depth  of  about  8  inches  and  flared  horizontally  to  a  con- 
siderable width  varying  from  about  20  to  40  feet.     Beneath 
the  suction  head  is  a  pressure  chamber  with  a  series  of 
nozzles,  through  which  water  is  pumped  under  pressure, 
constituting  the  jet  agitator,  the  province  of  which  is  to 
loosen  the  sand  or  other  material  for  entrance  into  the 
suction. 

4.  Smaller    Maximum   Depth    Capable   of  being   Made. 
Mississippi  River  Dredges  are  generally  equipped  to  lower 
their  suctions  to  a  depth  not  exceeding  20  feet,  and  the 


88  DREDGING  ENGINEERING 

practice,  apparently,  is  to  dig  always  as  deep  as  the  suction 
head  will  permit. 

5.  Use  of  Ihe  Double  Suction. — Most  of  the  dredges  built 
by  the  Commission  have  a  single  dredge  pump  set  verti- 
cally in  the  plane  of  the  longitudinal  axis  of  the  hull,  with  a 
double  suction  leading  into  it  on  both  the  port  and  star- 
board side,  and  a  single  discharge  pipe  extending  axially 
from  the  pump  to  the  stern  of  the  dredge,  thence  to  the 
pontoon  line.  Each  suction  is,  roughly,  24  inches  in  diam- 
eter, and  the  discharge  pipe  from  32  to  36  inches.  In- 
cidentally, the  balanced  suction  obviates  the  necessity 
for  a  thrust  bearing. 

As  in  the  other  river  types,  the  upper  deck  contains 
quarters  for  the  captain  and  crew,  numbering  generally 
from  45  to  50  men. 

The  bars  to  be  dredged  form  as  a  rule  diagonally  across 
the  river  at  the  point  of  change  in  the  direction  of  curva- 
ture of  the  current,  so  that  they  lie  between  two  pools  of 
relatively  deep  water,  located  at  the  outer  bank  of  two  suc- 
cessive bends  in  the  river.  The  axis  of  the  dredged  cut 
should  coincide  with  the  direction  of  the  current,  using  a 
sufficient  length  of  pontoon  line  to  discharge  into  the  deep 
water  below  the  bar.  Successive  parallel  cuts  yield  the 
desired  channel  width. 

More  detailed  information  regarding  the  Mississippi 
River  Dredges  and  the  performance  tests  made  by  the 
Commission  is  contained  in  a  paper  by  F.  B.  Maltby, 
M.  Am.,  Soc.  C.  E.,  published  in  Vol.  LIV.,  of  the 
Transactions. 


CHAPTER  VII 

HYDRAULIC  DREDGES  OF  THE  SEA-GOING 
HOPPER  TYPE 

Historical. — In  the  improvement  and  maintenance  of 
important  harbors  and  entrance  channels,  involving  the 
dredging  of  ocean  bars  in  exposed  locations,  it  was  early 
realized  by  the  U.  S.  Government  through  the  Corps  of 
Army  Engineers,  in  charge  of  all  such  work,  that  the 
most  economic  method  of  attack  was  by  government-built 
and  operated  dredges  of  large  capacity,  capable  of  remov- 
ing great  quantities  of  material  within  a  reasonable  time 
at  low  cost.  This  conclusion  was  reached  after  repeated 
failures  of  dipper  and  grapple  dredges  economically  to 
cope  with  the  situation.  With  such  types  one  of  two 
equally  undesirable  results  was  experienced:  either  the 
dredges  and  the  numerous  items  of  appurtenant  floating 
plant  were  rapidly  destroyed  by  the  continual  pounding 
against  each  other,  or  else  the  work  proved  slow  and 
expensive  because  of  the  large  precentage  of  lost  time 
waiting  for  calm  weather.  Since  larger  and  more  suitable 
dredges  were  not  to  be  had  of  the  contractors,  the  Govern- 
ment, apparently  adopted  the  only  alternative,  and  since 
1890  more  than  twenty  seagoing  suction  dredges  have 
been  built  by  them. 

The  use  of  this  type  of  dredge  in  the  United  States  dates 
as  far  back  as  1855.  Quoting  from  Major  J.  C.  Sanford's 
very  excellent  paper  on  " Dredging  Ocean  Bars,"  printed 
in  Vol.  LIV.  Transactions  Am.  Soc.  C.  E.,  "The  earliest 
dredge  of  this  type  used  in  the  United  States  was,  it  is 
believed,  the  GEN.  MOULTRIE,  used  in  1855,  at  Charleston, 
S.  C.  ...  This  dredge  is  stated  to  have  been  a  moderate 
sized  commercial  steamboat,  converted  into  a  dredge  by 
the  addition  of  centrifugal  dredging  pumps  witlTnecessary 

89 


90 


DREDGING  ENGINEERING 


piping,  etc.,  and  with  bins  constructed  in  the  hold.  Her 
drags,  or  suction  heads,  were  probably  somewhat  similar 
to  those  now  in  use.  .  .  .  From  25  cents  to  $1.00  per 
cu.  yd.  for  material  dredged,  according  to  locality  and 
amount  of  work,  was  still  (previous  to  1890)  considered 
a  fair  price  for  dredging  on  ocean  bars;  and  on  account  of 
the  great  cost  of  dredging,  the  plan  of  deepening  these  bars 
by  scour  produced  by  jetties  was  regarded  as  the  only 
practicable  one." 


FIG.  36. — The  Comstock.     (Courtesy  the  Bucyrus  Co.) 

However,  after  the  conclusion,  in  1891,  of  six  years  of 
dredging,  by  contract,  to  deepen  the  entrance  channel  to 
New  York  Harbor,  "the  cost  of  maintaining  the  dredged 
channel  to  full  width  and  depth  has  been  found  to  be  very 
moderate." 

The  first  suction  hopper  dredge  built  by  the  Government 
was  the  CHARLESTON,  in  1890,  having  one  15  inch  pump  and 
a  bin  capacity  of  340  cu.  yds.  Between  1890  and  1900, 


DREDGES  OF  THE  SEA-GOING  HOPPER  TYPE  91 

three  more  were  built,  the  performance  of  which  effected 
such  a  reduction  in  the  cost  of  dredging  ocean  bars  as  to 
question  the  economy  of  constructing  the  costly  jetty 
for  the  purpose  of  deepening  these  channels.  Since  1900, 
the  Government  has  built  a  goodly  fleet  of  these  machines, 
probably  the  most  intensive  construction  period  being  from 
1901  to  1904,  during  which  the  total  number  constructed 
and  under  construction  was  14.  Two  such  dredges,  the 
CARIBBEAN  and  CULEBRA,  built  in  1907  by  the  Maryland 
Steel  Company,  of  Baltimore,  Md.,  were  used  in  the  con- 
struction of  the  Panama  Canal. 

General  Description. — The  sea-going  hydraulic  dredge 
is  a  self  propelled  vessel  with  moulded  hull,  containing  one 
or  two  main  pumping  plants,  and  with  hoppers  for  receiv- 
ing and  transporting  the  material.  In  addition  to  the 
apparatus  required  solely  for  the  dredging  operation,  it  is 
equipped  with  all  the  machinery  and  appliances  necessary 
to  ocean  navigation. 

The  hull  is  generally  steel  with  a  hopper  capacity  of 
from  2000  to  3000  cubic  yards,  and  from  200  to  300  feet 
long.  Some  types  have  but  one  dredge  pump  and  one 
suction  pipe  operating  through  a  central  well.  The  ma- 
jority, however,  have  two  pumps  arid  two  suctions,  as 
experience  has  proven  the  advantage  of  the  dual  installa- 
tion. The  suction  pipe  is  hinged  at  the  pump,  and  raised 
and  lowered  by  tackle.  The  joint  must  be  flexible  to  allow 
a  certain  amount  of  hull  motion  without  disrupting  the 
pipe.  The  suction  end,  resting  on  the  bottom  is  provided 
with  a  scraper  or  shoe  to  feed  the  pump.  In  operation, 
the  dredge  travels  forward  at  a  speed  of  about  6  knots, 
with  the  suction  head  or  shoe  dragging  on  the  mud.  The 
material  is  distributed  evenly  in  the  bins  through  pipes  and 
distributing  chutes.  The  large  amount  of  accompanying 
water  is  drained  off  by  overflow  through  the  sides,  or  over 
raised  coamings.  When  loaded,  the  dredge  steams  to  the 
dumping  ground  and  discharges  the  hoppers  through  gates 
on  the  bottom,  which  are  operated  by  a  vertical  engine- 
driven  worm  or  similar  arrangement.  Some  dredges  are 


92  DREDGING  ENGINEERING 

equipped  to  empty  their  bins  by  means  of  their  dredge 
pumps.  American  practice  tends  to  two  large  bins,  one 
forward  and  one  aft  of  the  machinery  space. 

There  are  two  details  which  merit  especial  attention, 
as  a  proper  consideration  of  them  is  so  necessary  to  the 
success  of  the  unit.  They  are  the  drag — or  scraper  or  shoe 
— and  the  flexible  member  of  the  suction  for  the  purpose  of 
accommodation  to  the  pitching  and  rolling  of  the  steamer. 
Of  the  former,  many  types  have  been  used  with  varying 
success  in  different  materials.  No  one  design  can  be  ac- 
cepted as  a  standard  for  all  conditions  The  latter  usually 
consists  of  a  section  of  rubber  pipe  about  10  feet  long,  in- 
serted in  the  suction  near  the  elbow. 

A  feature  of  the  later  suction  hopper  dredges  is  the  in- 
troduction of  overflows  at  different  elevations  in  the 
hoppers,  in  order  to  reduce  the  load  draft  when  working  in 
shallow  water.  Although  the  use  of  the  lower  overflow 
necessarily  effects  a  reduction  in  bin  capacity,  yet  it  is 
regarded  as  a  permanent  improvement,  increasing  the 
dredge's  scope  and  adaptability. 

Advantages  of  the  Type — Mr.  Sydney  B.  Williamson, 
Chief  Engineer,  Pacific  Division,  Panama  Canal,  writes: 

"This  type  of  dredge  has  proven  decidely  the  most  economical  for 
the  work  of  improving  and  maintaining  the  harbors  and  channels  of 
our  coast,  and  the  superiority  is  largely  due  to  its  being  self-contained, 
self-propelling  and  possessing  the  ability  to  operate  without  anchorage." 

While  the  machine  is  particularly  adapted  to  ocean  bar 
dredging,  which  is  the  work  for  which  it  was  originally 
designed,  it  presents  certain  advantages  for  maintenance 
work  in  rivers  and  sheltered  harbors.  Because  it  exca- 
vates the  shoal  progressively  from  the  top  downward,  in- 
stead of  from  the  end,  it  is  economical  of  operation  in 
shallow  cutting  and,  moreover,  cooperates  with  the  deepen- 
ing tendencies  of  current  scour.  Requiring  no  anchorage, 
it  offers  no  obstruction  to  navigation. 

On  the  other  hand,  it  is  open  to  criticism  in  two  partic- 
ulars, viz.,  the  discontinuity  of  dredging  operation  due 
to  frequent  trips  to  the  dump,  and,  secondly,  the  large 


DREDGES  OF  THE  SEA-GOING  HOPPER  TYPE  93 

proportion  of  unremunerative  pumping  because  of  the 
material  carried  away  in  suspension  in  the  hopper  effluent, 
which,  of  course,  is  much  more  true  of  alluvial  silt  than  of 
material  of  greater  specific  gravity.  The  U.  S.  Dredge 
DELAWARE,  operating  on  Duck  Creek  flats  on  the  Delaware 
River,  in  1908,  found  that  it  was  impossible  to  fill  the  bins 
with  solid  material,  owing  to  its  very  soft  nature,  and  the 
desired  results  were  accomplished  by  using  the  bins  on  the 
flood  tide  only,  pumping  directly  overboard  on  the  ebb 
tide  so  that  the  liquid  material  was  carried  by  the  current 
to  deep  water  in  the  bay.  This  plan  of  operation  con- 
tinues in  use  to-day  and  is  called  the  "agitation"  method. 

TYPICAL  EXAMPLES  DELAWARE  CULEBRA 

When  built 1906  1907 

Length  overall 315'  288' 

Beam  (moulded) 52'  47H 

Depth  (moulded) 22>£'  25 

Bin  capacity — cu.  yds 3000  2300 

No.  of  pumps 2  2 

Size  of  pumps 20"  20" 


PART  II 

DREDGING 


CHAPTER  VIII 
OBJECTS  AND  PHASES  OF  THE  SUBJECT 

Our  first  thought  as  to  the  "why"  of  Dredging  is  the 
creation  of  water  depths  in  excess  of  the  natural,  nor  have 
our  mental  processes  erred  in  the  spontaneous  answer. 
Yet  there  are  several  other  important  purposes  of  the 
art  and  many  reasons  for  increasing  the  depth  of  water. 
The  Objects  of  Dredging  may  be  included  broadly  under 
four  headings: 

1.  The  creation  of  water  depths  in  excess  of  the  natural. 

2.  The  acquisition  of  subaqueous  material  for  use  as  fill. 

3.  The  construction  of  dikes  and  levees. 

4.  The  acquisition  of  subaqueous  material  for  its  com- 
mercial value. 

The  first  may  be  desired  for  the  navigation  of  ships;  for 
harbors;  for  the  control  of  rivers  and  other  water-ways; 
for  construction  purposes  in  connection  with  piers,  wharves, 
docks,  shipways,  dams  and  subaqueous  foundations  gener- 
ally; and  for  drainage  and  irrigation. 

The  second  refers  to  the  utilization  of  the  "dredgings, " 
or  dredged  material,  for  land  reclamation;  for  the  rehabili- 
tation of  swamp-lands  and  marshes;  for  filling  litoral 
structures;  and  for  "making  bottom,"  i.e.,  the  deposition 
of  heavy  material  upon  a  soft  mud  bottom  to  displace 
and  compact  the  mud  with  the  idea  of  increasing  the  bearing 
power  and  lateral  stability  of  the  original  yielding  material. 

The  third  heading  covers  the  use  of  the  dredge  as  the 
sole  or  appurtenant  agent  in  the  construction  of  dikes 

95 


96  DREDGING  ENGINEERING 

for  river  control  and  for  impounding  basins  and  land 
reclamations. 

The  fourth  comprises  the  dredging  of  sand,  gravel  and 
clay  for  building  purposes;  of  phosphate  rock  for  fertilizer; 
of  gold  and  other  metals;  and  even  of  subaqueoife  coal  in 
some  localities. 

Finally,  a  dredging  operation  may  have  a  dual  function, 
such  as  making  depth  and  building  up  a  fill  at  the  same 
time. 

It  is  beyond  the  scope  of  this  book  to  treat  of  dredging 
in  all  its  branches  as  summarized  above  but  rather  will 
the  discussion  apply  to  excavation  for  depth  and  the  useful 
disposal  of  the  dredged  material  in  making  fills. 

A  dredging  project  of  reasonable  magnitude  will  usually 
have  three  successive  phases  or  stages  and  we  will  present 
the  subject  so  divided,  believing  that  both  the  readers' 
grasp  and  our  exposition  will  be  facilitated  thereby.  They 
are  first,  the  Preliminary  Engineering,  by  which  the  site 
is  explored  and  the  work  laid  out;  second,  the  Preliminary 
Construction  of  appurtenant  structures  that  must  be  built 
before  actual  dredging  can  be  commenced,  such  as  dikes, 
sluiceways  and  pipe  lines;  and  third,  the  Operation,  when, 
the  stage  set  and  the  plant  assembled  and  working  upon 
the  predetermined  program,  the  job  becomes,  by  natural 
growth,  a  purely  operating  problem. 


CHAPTER  IX 
PRELIMINARY  ENGINEERING 

Exploration  of  Site/ — The  initial  step  in  any  proposed 
dredging  operation  is  the  examination  of  the  physical 
properties  of  the  site  of  the  work,  and  often  of  the  adjacent 
terrain  as  well,  for  the  determination  of  the  location  and 
outline  of  the  area  to  be  dredged,  the  quantity  and  nature 
of  the  material  to  be  removed  to  meet  the  requirements, 
and  the  selection  of  impounding  basin  sites  for  the  disposal 
of  the  dredgings.  This  is  accomplished  by  a  survey, 
partly  topographic  as  well  as  hydrographic,  and  by  borings. 

As  a  basis  for  the  survey,  a  set  of  coordinates  is  estab- 
lished with  reference  to  a  convenient  origin,  usually  tied 
in  to  the  War  Department  Engineers'  stations,  and  a  base 
line  and  triangulation  system  are  laid  off.  From  the  main 
triangulation  stations,  secondary  stations  are  located  at 
points  from  which  soundings  may  be  located  conveniently 
by  instrument  intersection.  Sounding  parties,  each  con- 
sisting of  a  chief  of  party,  two  instrument  men,  a  leadsman, 
a  recorder  and  a  boatman,  are  then  sent  out.  A  gauge, 
to  measure  the  varying  elevation  of  the  water  surface,  is 
set  with  its  zero  at  the  datum  used,  and  a  gauge-reader 
observes  and  records  the  readings  thereon  at  stated  time 
intervals.  The  same  results  may  be  obtained  automatically 
by  the  use  of  an  instrument  called  a  hydrochronograph, 
which  is,  in  principle,  a  cylinder  rotated  by  a  clock  and 
wrapped  with  cross  section  paper,  upon  which  a  stylus, 
rising  and  falling  with  the  water,  traces  the  curve  of  vary- 
ing surface  elevation.  If  in  tidal  waters,  the  gauge  will 
be  a  tide-gauge  with  its  zero  at  mean  low  water,  and, 
since  all  soundings  are  referred  to  the  datum,  the  gauge 
record  must  be  made  whenever  soundings  are  being  taken. 
Ranges  are  established  indicating  the  lines  of  soundings. 
7  97 


98  DREDGING  ENGINEERING 

As  the  boat  containing  the  leadsman,  recorder,  boatman 
and  usually  the  chief  of  party,  traverses  a  range,  the  two 
transitmen  on  shore,  set  up  on  known  stations,  locate  it 
by  simultaneous  angle  readings  upon  signal  from  the  boat. 
It  is  customaty  to  signal  at  a  constant  time  interval, 
perhaps  of  one  minute.  The  leadsman  heaves  the  lead  con- 
stantly, calling  off  the  depths  to  the  recorder,  while  the 
boatman  keeps  the  craft  on  range  and  varies  his  speed  with 
the  depth  of  water  so  that  the  soundings  may  be  a  more 
uniform  distance  apart.  That  sounding  which  is  taken 
just  as  the  signal  is  given  the  transitmen  to  read,  is  termed 
"on  the  cut-off"  and  is  so  marked  in  the  book.  The 
draughtsman  in  the  office  first  plots  these  " cut-offs"  and 
then  divides  the  straight  line  connecting  each  pair  into 
equal  parts  for  the  intermediate  soundings.  The  sound- 
ings may  be  obtained  as  close  as  desired  by  regulating  the 
speed  of  the  boat  and  the  distance  between  ranges.  The 
engineer  in  charge  should  use  some  judgment  as  to  the 
proper  number  of  soundings  commensurate  with  the  given 
requirements.  When  the  entire  area  has  been  covered, 
the  final  sounding  sheet  is  prepared,  showing  also  the  coor- 
dinates and  contours. 

As  the  direction  and  velocity  of  flow  will  very  often  have 
an  important  bearing  upon  the  proper  location  of  a  channel, 
particularly  as  to  the  maintenance  thereof,  it  may  be  nec- 
cessary  to  obtain  some  data  in  this  regard.  Taking 
" current-drift, "  as  it  is  called,  is  accomplished  as  follows: 
A  number  of  floats  are  made,  deep  enough  to  represent 
the  average  flow  and  almost  entirely  submerged  to  reduce 
the  wind  influence  to  a  minimum.  They  are  released  at 
intervals  in  the  cross  section  of  the  water  way  and  per- 
mitted to  take  their  own  courses.  The  chief  of  party, 
in  a  motor  boat,  runs  from  float  to  float,  signalling  by  flag 
to  two  transitmen  on  shore,  who  read  simultaneously  upon 
signal.  Each  instrument  man  also  records  the  time  of 
each  reading,  their  watches  having  been  set  in  agreement. 
The  angular  intersections  are  plotted  and  the  time  of  each 
shown,  so  that  the  broken  line  connecting  successive 


PRELIMINARY  ENGINEERING  99 

positions  of  each  float  shows  the  path  taken  and  the  dis- 
tance travelled  in  a  stated  time.  The  current-drift  trac- 
ing and  the  sounding  chart  should  be  drawn  to  the  same 
scale  that  they  may  be  superimposed  to  determine  the 
relation  between  depth  and  flow. 

The  character  of  the  bottom  strata  is  investigated  either 
by  probings  or  borings.  By  probing  is  meant  simply 
" feeling  out"  the  bottom  with  a  pointed  rod  or  pipe,  which 
is  thrust  into  and  worked  down  through  the  soil  by  two 
or  three  men  or  by  a  maul  or  monkey.  The  depth  of  pene- 
tration is  an  indication  of  the  hardness  of  the  bottom,  and 
an  experienced  man  can  tell,  within  limits,  the  nature  of  the 
strata  by  the  feel  of  the  probe  as  he  works  it.  The  results 
obtained  are  neither  so  complete  nor  so  reliable  as  the 
information  yielded  by  borings,  and,  unless  taken  by  a 
first-class  man,  may  even  be  misleading.  The  only  fea- 
tures of  probing  that  can  be  cited  as  recommendations  for 
its  use  are  inexpensiveness  and  convenience,  with  the 
reservation  that  the  time  and  money  saved  may  easily 
prove  a  false  economy.  For  purposes  other  than  the 
determination  of  the  nature  of  the  material  to  be  dredged, 
however,  such  as  locating  the  contour  of  the  underlying 
bed  rock  by  driving  a  heavy  probe  with  a  power  driven 
drop  hammer  of  light  proportions,  probing  may  give  fairly 
accurate  results,  and,  in  such  employment,  the  above 
criticism  is  not  entirely  applicable. 

Borings  differ  from  probings  principally  in  that,  with 
the  former,  samples  of  the  material  are  obtained.  There 
are  two  principal  kinds  of  borings,  wash  and  core  borings, 
although  a  combination  of  the  two  methods  will  often  effect 
an  economy.  Various  makeshifts  have  been  tried  with 
more  or  less  success  for  bringing  samples  of  the  bottom 
material  to  the  surface  for  examination,  such  as  thrusting 
an  open  pipe  into  the  soil  and  removing  the  contents  thereof 
after  raising  it  clear  of  the  water  and  again  by  the  use  of  a 
wood-auger.  The  true  boring,  however,  whether  wash  or 
core,  implies  the  use  of  a  casing,  consisting  of  sections  of 
heavy  pipe  about  3  inches  in  diameter.  In  the  wash 


100  DREDGING  ENGINEERING 

boring  the  active  instrument  consists  of  a  hollow  drill  rod, 
a  tube  or  pipe  of  smaller  diameter,  having  at  its  lower  end  a 
chopping  bit  with  an  X-shaped  chisel  point  and  with 
openings  for  a  water  jet.  The  drill  rod  is  worked  down 
inside  the  casing  by  rotating  it,  or  by  raising  and  dropping 
it  through  short  heights,  or  by  the  two  motions  combined. 
At  the  same  time,  water  is  forced  down  the  hollow  drill 
rod  by  a  force  pump,  and,  escaping  through  the  jet  holes 
in  the  bitt,  carries  upward  the  loose  material  in  the  annular 
space  between  the  rod  and  casing.  Simultaneously,  the 
casing  is  also  worked  down  by  rotation  or  by  driving. 
The  overflow  from  the  top  of  the  casing  is  caught  in  a 
bucket  and  allowed  to  settle,  and  the  samples  therefrom 
are  preserved  in  bottles  duly  labelled.  The  apparatus 
is  handled  by  a  derrick  frame  and  hoist  mounted  on  a  scow 
or  by  a  pile  driver. 

While  the  term,  "core  boring, "  is  generally  applied  to 
the  use  of  core  drills  with  diamond  bitts  or  chilled  steel  or* 
toothed  cutters  such  as  are  used  in  testing  rock  strata,  the 
drill  being  sunk  by  rapid  rotation  and  raised  at  intervals 
for  the  removal  of  the  core  contained  within  it,  core  speci- 
mens may  be  obtained  in  connection  with  the  use  of  the 
wash-boring  apparatus  described  above  and  will  generally 
suffice  for  dredging  data,  obviating  the  use  of  the  more 
expensive  equipment.  This  is  done  by  substituting  for 
the  drill  rod  a  short  piece  of  brass  pipe  which  is  pressed  into 
the  bottom  within  the  casing  and  lifted  out  for  examination, 
or,  in  hard  material,  a  saw-tooth,  tubular  bit  may  be  used. 
The  operation  is  performed  in  the  dry  and  procures  core 
samples  of  the  material  in  its  natural  condition.  The 
results  of  a  wash  boring  are  possible  of  misinterpretation 
due  to  the  fact  that  the  samples,  having  been  jetted  and 
churned,  are  not  indicative  of  the  true  stratum  relation 
and  due  also  to  the  tendency  of  the  jet  to  wash  up  only  the 
finer  material.  It  is  advisable  therefore  to  obtain  dry 
core  samples  whenever  feasible. 

Estimating  the   Quantities. — The   engineer  is   now   in 
shape  to  locate  definitely  the  area  to  be  dredged,  to  esti- 


PRELIMINARY  ENGINEERING  101 

mate  the  yardage  to  be  removed,  to  choose  the  type  of 
plant  and  method  of  disposal  of  the  dredgings,  to  approxi- 
mate the  cost  and  finally  to  let  the  contract  for  the  work. 

The  location  of  the  area  to  be  dredged,  if  not  definitely 
fixed  by  local  conditions,  may  involve  the  considerations 
of  sedimentation,  scour,  bank  maintenance,  dikes  or  other 
factors  of  waterway  control,  which  will  be  discussed  more 
fully  in  a  later  chapter. 

The  quantity  of  material  to  be  removed  may  be  com- 
puted in  several  ways.  First  and  most  common  is  the 
method  of  vertical  cross  sections  at  regular  intervals,  from 
which  the  volume  is  obtained  by  average  end  areas  or  by 
the  prismoidal  formula.  The  second  is  the  "planimeter" 
method.  Each  contour  is  traversed  by  the  planimeter  to 
find  the  included  area,  resulting  in  a  series  of  areas  of 
equidistant  horizontal  planes  from  which  the  volume 
follows  as  above.  A  third  method,  which  might  be  termed 
the  "Unit"  scheme,  divides  the  area  into  a  number  of 
squares  of  convenient  size  by  ruling  the  plan  with  hori- 
zontal and  vertical  lines,  uniformly  spaced.  From  the 
average  depth  of  cut  in  each  square,  the  volume  of  dredging 
therein  is  computed  and  noted  on  the  drawing.  The  first 
method  is  best  adapted  to  long,  narrow  areas  such  as 
channels,  canals,  docks,  etc.,  and  the  second  and  third 
to  large  polygonal  areas. 

To  simplify  the  arithmetic  involved,  the  following 
arrangements  are  useful.  In  the  calculation  of  the  volume 
by  the  method  of  average  end  areas,  let  A\,  Az,  As,  .  .  . 
An  represent  the  areas  of  successive  cross  sections,  D  the 
constant  distance  between  sections,  and  V,  the  desired 
volume.  Then 

7  = 


2 
D 

=  0  [Ai  +  An  +  2  (A2  +  A,  +  A*  +. .    .  An_i)] 


102 


DREDGING  ENGINEERING 


Thus,  having  listed  the  areas  of  all  the  sections,  add  the 
first  and  last  to  twice  the  sum  of  the  others  and  multiply 
the  grand  total  by  one-half  the  common  distance  between 
sections.  The  same  principle  is  applicable  to  the  deter- 
mination of  the  areas  of  the  sections,  substituting  or  din  at  es 
for  areas. 


FIG.  37.  —  The  unit  method. 


For  volume  computation  by  the  prismoidal  formula,  a  set 
of  multipliers  is  used  analogous  to  Simpson's  multipliers 
for  finding  ship  displacements.  Using  the  same  notation 
as  above: 


The  volume  of  the  prism  AI  to  Az  =-«  (Ai  +  4A2  + 


A,  to  Ab  m  ~ 


and  V 


- 


The  multipliers,  therefore,  are  1  for  AI,  4  for  A2,  2  for  As, 
etc. 

In  computing  the  average  depth  in  each  square  of  the 
third,  or  unit  method,  the  weighted  mean  must  be  deter- 
mined. Assuming  that  the  soundings  are  sufficiently 
close  to  fix  9  depths  in  each  square,  whether  directly  or 
by  interpolation,  as  shown  in  Fig.  37,  page  102,  one  in  each 


PRELIMINARY  ENGINEERING  103 

corner,  one  in  the  center  of  each  ide  and  one  in  the  middle, 

the  average  depth  of  water  for  the  area  of  the  square  will  be 

A+C+G+J+2  (j^+  F  +  H  +  D)  +  4ff 

16 

The  volume  is  reduced  to  cubic  yards,  the  unit  of  measure 
in  dredging. 

The  question  of  the  degree  of  side  slope  that  the  sides 
of  a  dredged  cut  will  assume  is  important  and  will  have  a 
considerable  effect  upon  the  estimated  quantity  of  material 
to  be  removed.  It  is  essential  that  the  engineer  approxi- 
mate the  ultimate  inclination,  even  though  he  specify 
a  minimum  slope  as  the  limit  of  pay  quantities.  Correct 
estimates  are  necessary  to  the  fulfillment  of  schedules. 
Yet  it  is  impossible  to  classify  bottom  materials  upon  the 
basis  of  amount  of  slope,  since  the  combination  of  the  con- 
trolling factors,  nature  and  stratum  relation  of  the  mater- 
ials, depth  of  cut  and  current  influence,  is  never  the  same. 
It  is  advisable  to  supplement  his  judgment  based  upon  the 
results  of  the  borings  by  the  knowledge  of  experience  and 
by  available  local  data.  If  the  problem  permits  a  gen-' 
erality,  it  may  be  said  that,  ordinarily,  the  side  slope  of 
dredged  river  channels  will  vary  from  1  on  3  to  1  on  5 
(excepting  in  rock  excavations),  and  in  docks,  where  the 
piling  offers  some  support  to  the  material,  from  1  on  2  to 
1  on  4.  River  silt,  however,  is  often  so  semi-fluid  in  char- 
acter as  to  assume  a  slope  approaching  1  on  oc . 

The  actual  quantity  of  material  in  place  between  the 
original  bottom  surface  and  the  specified  depth  to  be  dred- 
ged is  modified  by  two  important  factors;  overdepth  dredging 
and  the  ratio  between  scow  and  place  measurement,  the  first 
being  applicable  to  all  dredging  and  the  second  to  scowed 
material  only.  It  is  manifestly  impossible  for  a  dredge 
to  excavate  precisely  to  the  plane  desired.  The  dredged 
bottom  will  be  more  or  less  irregular  no  matter  what  the 
type  of  machine  used,  but  more  particularly  with  the  use 
of  grapples,  of  which,  from  the  very  nature  of  their  mode 
of  removal,  it  must  be  expected  that  the  bottom  will  be 


104  DREDGING  ENGINEERING 

uneven  and  pitted  to  a  degree  depending  upon  the  kind  of 
material  and  the  skill  of  the  operator.  For  this  reason, 
it  is  customary  to  specify  a  certain  overdepth  allowance, 
or,  in  other  words,  the  adoption  of  a  second  plane,  a  short 
distance  below  the  specified  depth,  with  the  understanding 
that  the  yardage  unavoidably  removed  from  between  the 
two  shall  be  included  in  the  pay  quantities.  Thus,  no 
penalty  is  exacted  of  the  dredge  for  reasonable  overdepth 
dredging,  only  such  material  as  is  excavated  from  below 
the  overdepth  plane  being  deducted  from  the  total  yardage 
credited  to  the  machine.  In  the  language  of  the  U.  S. 
Engineer  Office,  Washington,  D.  C.,  in  specifications  for 
dredging  the  Potomac  and  Eappahannock  Rivers,  1917, 

"To  cover  mechanical  inaccuracies  of  dredging  processes,  material 
actually  removed  to  a  depth  of  not  more  than  1  foot  below  the  required 
depth  will  be  estimated  and  paid  for  at  full  contract  price;  to  secure 
stable  banks  for  the  dredged  cut,  material  which  is  actually  removed 
on  order  of  the  contracting  officer  in  quantities  sufficient  to  make  side 
slopes  not  flatter  than  one  (1)  on  three  (3),  whether  dredged  in  situ  or 
after  having  fallen  into  the  cut,  will  be  estimated  and  paid  for.  The 
allowance  for  overdepth  on  the  specified  slopes  will  be  the  same  as  in 
the  channel  and  will  be  measured  vertically.  Material  taken  from  be- 
yond the  limits  above  described  will  be  deducted  from  the  estimates 
as  excessive  overdepth  dredging  and  will  not  be  paid  for." 

For  grapple  and  dipper  dredges,  the  usual  overdepth 
allowance  is  two  (2)  feet,  and  for  ladder  and  hydraulic 
machines,  one  (1)  foot.  In  estimating  the  yardage  to  be 
removed,  it  is  good  practice  to  assume  that  all  the  allowed 
overdepth  dredging  will  be  removed,  or,  in  other  words,  to 
include  in  the  figure  representing  the  total  quantity  to  be 
dredged  all  the  material  lying  above  the  allowed  overdepth 
plane.  Conservatism  advises  full  rather  than  scant  esti- 
mates, since  upon  them  are  based  the  appropriations  and 
the  schedules. 

It  is  a  well  known  fact  that  dredged  material  occupies 
more  space  in  the  scows  than  "in  place"  in  the  original 
bottom,  but  it  is  often  a  moot  question  as  to  the  amount 
of  expansion  for  any  given  material.  Frequently,  the 
necessity  arises  for  converting  place  into  scow  measure- 


PRELIAtlNARY  ENGINEERING  105 

ment  and  vice  versa.  Such  of  his  place  quantities  as  are 
to  be  scowed,  the  engineer  increases  by  a  certain  percentage 
to  arrive  at  the  total  quantity  which  will  result  from  the 
number  of  scows  loaded  and  the  capacity  of  each  scow, 
determined  from  the  inside  dimensions  of  the  pockets. 
Material  that  is  bucket  dredged  into  scows  is  nearly  always 
paid  for  by  the  cubic  yard  scow  measure,  but,  whether  the 
basis  of  compensation  be  scow  or  place  measurement, 
occasions  are  prone  to  arise  for  the  conversion  of  one  to  the 
other,  even  in  the  pay  quantities  and,  for  that  reason,  the 
relation  between  them  is  often  fixed  by  the  specifications. 

Omitting  rock,  the  percentage  of  increase  over  place 
quantities  will  vary  from  5  to  25  per  cent,  depending  upon 
the  nature  of  the  material.  Since  the  expansion  is  due  to 
the  presence  of  voids  in  the  contents  of  the  scow  in  conse- 
quence of  the  breaking  or  cutting  up  of  the  monolithic 
mass  of  the  original  bottom,  that  material  of  which  the 
separate  bucket  loads  most  nearly  retain  their  shapes  as 
dug  will  show  the  greatest  volumetric  increase.  Such 
materials  are  blue  clay  and  stiff  mud  and  their  percentage 
of  increase  should  be  taken  as  about  25  per  cent.  Going 
to  the  other  extreme,  those  dredgings  which  are  almost 
semi-fluid  in  character,  such  as  river  silt  and  find  sand,  and 
tend  to  run  together  in  the  pocket,  with  a  resultant  solidity 
more  nearly  approaching  that  of  their  original  "in  situ" 
condition,  are  increased  in  volume  in  the  scows  only  about 
5  per  cent.  Between  these  two  limits,  5  and  25  per  cent, 
other  materials  and  combinations  of  materials  will  range 
according  to  their  "body"  or  "cohesive  stamina."  For 
instance,  the  proper  figure  for  a  fairly  heavy  equal  part 
mixture  of  sand  and  mud  appears  to  be  about  16%  per  cent, 
or  one-sixth.  In  the  majority  of  dredging  operations,  other 
than  those  involving  the  removal  of  loose,  rotten  and  bed 
rock,  the  quantity  of  material  measured  in  place  must  be 
increased  by  from  15  to  25  per  cent,  to  obtain  the  number 
of  cubic  yards  scow  measurement  of  the  same  material. 
Again  quoting  the  U.  S.  Engineer  Office,  Washington,  D.  C. 


106  DREDGING  ENGINEERING 

from  specifications  for  dredging  in  the  Potomac  and  Rappa- 
hannock  Rivers,  1917, 

"When  necessary  for  any  cause  to  convert  scow  measurement  into 
place  measurement  or  the  reverse,  100  cubic  yards  of  the  former  will  be 
taken  as  the  equivalent  of  80  cubic  yards  of  the  latter." 

Here  the  material  was  defined  as  "  freshet  deposit  .  .  . 
composed  of  sand  and  mud." 

Deck  loads  of  stone  or  rock  are  measured  either  by  cross 
sectioning  the  pile  or  by  displacement  calculations  from 
readings  of  the  light  and  load  draughts  of  the  lighter  upon 
glass  tubes  set  in  the  four  corners  of  the  hold. 

If  the  object  of  the  dredging  is  the  creation  of  a  depth 
sufficient  for  the  flotation  of  large  steamships,  there  are 
several  pertinent  factors  to  be  considered  in  the  determina- 
tion of  that  depth.  The  question  is  how  much  deeper  than 
the  load  draft  of  the  vessel  at  rest  must  the  dredging  be 
carried.  In  berths,  adjacent  to  piers  and  wharves,  ob- 
viously it  is  necessary  merely  that  there  be  sufficient  water 
to  float  the  loaded  vessel  at  times  of  extreme  low  water. 
It  is  often  advisable,  however,  particularly  where  the 
bottom  material  is  of  a  hard  resistant  nature  and  prone  to 
irregularity,  that  an  additional  foot  or  two  be  provided 
as  a  factor  of  safety,  but  where  the  material  is  so  soft 
and  free  of  obstruction  as  to  permit  a  reasonable  " nesting" 
of  the  vessel  to  it  without  injury,  this  may  not  be  necessary. 
The  increasing  tendency  to  ships  of  small  bilge  radius  and 
dead  rise,  culminating  in  the  fabricated  ships  of  almost 
rectangular  midships  section  requires  that  the  dredged 
depth  of  the  slip  be  practically  as  great  immediately  adja- 
cent to  the  pier  or  wharf  structure  as  elsewhere. 

In  ship  channels  traversed  by  large  steamships  under 
full  speed,  another  factor  must  be  considered,  viz.  the  in- 
creased draft  of  the  vessel  due  to  her  motion.  This  so- 
called  " squatting"  of  moving  vessels  has  been  thoroughly 
investigated  by  instrumental  observation  from  the  shore, 
the  results  of  which  prove  conclusively  that  the  amount 
of  "squatting"  is  of  such  magnitude  as  to  require  consid- 
eration. In  many  instances,  the  slight  grounding  of 


PRELIMINARY  ENGINEERING  107 

moving  vessels  in  the  channels  of  New  York  Harbor  where 
their  draft  at  the  pier  was  less  than  the  presumed  channel 
depth  has  been  shown  to  be  due  to  this  squatting  action 
instead  of  to  the  presence  of  shoals  as  at  first  thought. 
The  study  of  the  problem  has  lead  to  the  following  con- 
clusions: that  the  " squatting"  is  apparently  greater  in 
shoal  water  than  in  deep,  or  in  other  words  that  the  amount 
of  "  squat"  is  an  inverse  function  of  the  depth  of  water 
below  the  keel;  that  it  is  greater  at  high  speeds,  or  that 
its  amount  is  some  direct  function  of  the  speed;  that,  in 
the  majority  of  cases,  it  is  greater  aft  than  forward,  more 
particularly  at  high  speed  in  shoal  water;  and  that  it 
probably  varies  with  the  lines  of  the  hull.  A  " squat" 
of  as  much  as  four  feet  has  been  observed  upon  a  large 
steamship  traveling  at  high  speed  in  shoal,  calm  water, 
and  it  has  been  shown,  with  little  fear  of  contradiction, 
that,  under  the  conditions  most  favorable  to  a  great  a- 
mount  of  "  squat, "  large  ships  may  require  at  least  four 
feet  more  depth  in  the  channel  than  indicated  by  their 
drafts  at  the  pier.  The  " squatting"  aft  may  be  attri- 
buted to  the  hollow  under  the  ship's  stern,  but  it  is  not 
easy  to  perceive  why  the  ship  should  "squat"  at  the  bow. 
Choice  of  Plant  and  Method  of  Disposal  of  Dredgings.— 
The  question  of  the  kind  of  plant  best  adapted  to  a  par- 
ticular piece  of  work  is  of  the  utmost  importance  and  one 
in  which  a  mistake  may  entail  heavy  loss  in  time  and  money. 
Whether  the  decision  is  the  responsibility  of  the  engineer 
or  is  left  to  the  bidding  contractors,  it  should  be  given  due 
consideration  by  men  of  ripe  experience.  It  is,  of  course, 
intimately  related  to  the  problem  of  the  disposal  of  the 
dredgings,  and  must  be  discussed  in  conjunction  therewith. 
The  factors  involved  are  as  follows : 

(a)  Nature  of  the  soil 

0)   Proximity  of  a  free  dump 

(c)  Cost,  proximity  and  accessibility  of  impounding  basins,  and  the 
enhanced  value  of  impounding  basin  property, 

(d)  Kind  of  plant  available  and  the  relative  efficiency  of  units  of  the 
same  type, 


108  DREDGING  ENGINEERING 

(e)   Space  limitations  and  interference  with  construction  and  traffic, 
(/)   Depth  limitations, 
(g)  Time  limitations, 

Let  us  discuss  them  in  the  order  named. 

(a)  The  first  leads  to  a  comparison  of  the  various  types 
of  dredge  as  to  their  relative  merit  in  different  soils.  Ameri- 
can and  European  engineers  differ  in  their  preference 
of  dredge  types.  The  ladder  dredge  enjoys  considerable 
popularity  abroad,  while  in  this  country  the  grab  and  dipper 
are  much  more  extensively  used.  It  is  interesting  to 
note  the  foreign  and  local  expressions  of  opinion  on  the 
subject.  Brysson  Cunningham,  an  Englishman,  in  his 
"Dock  Engineering, "  while  admitting  that  the  ladder 
"  dredger "  is  not  economical  of  power  because  of  the  in- 
ordinately high  lift  necessary  to  feed  the  chutes,  never- 
theless strongly  commends  it,  writing  that  it  is  the  only 
machine  that  can  satisfactorily  dig  rock  and  is  excellent  in 
excavating  stiff  clay.  He  disposes  of  the  dipper  by  the 
simple  statement  that  "The  Dipper  Dredge  is  almost 
exclusively  an  American  type,"  and  of  the  grapple,  writes 
as  follows: — "The  grab  is  an  excellent  tool  and  invaluable 
in  confined  situations,  but  it  is  scarcely  suitable  for  general 
adoption  in  works  on  a  large  scale.  It  is  not  an  economical 
instrument  for  the  removal  of  stiff  clay;  its  best  perform- 
ances are  in  regard  to  mud  and  soft  earth.  It  cannot  be 
counted  upon  to  work  with  the  same  regularity  and  even- 
ness as  the  ladder  dredger;  in  fact,  its  tendency  is  to  pit  the 
surface  of  the  ground  with  a  series  of  hollows  and  depressions. 
But,  in  spite  of  these  drawbacks,  it  has  demonstrated  its 
ability  to  such  an  extent  that  it  is  looked  upon  as  an  essen- 
tial accompaniment  of  most  dock  and  harbor  undertakings." 
Even  this  may  be  regarded  as  quite  a  concession,  when  we 
remember  that  the  usual  English  grab  is  simply  one  or  two 
revolving  cranes  mounted  upon  a  scow. 

Prelini  (American)  in  his  "Dredges  and  Dredging, " 
expresses  American  feeling  in  a  nut-shell  in  the  statement, 
"Dippers  are  an  American  product,  and  the  marvel  is  that 
they  do  not  attain  wider  recognition  abroad  in  lieu  of  the 
expensive  ladder  dredges." 


PRELIMINARY  ENGINEERING  109 

Fowler  (American)  in  his  "  Practical  Treatise  on  Sub- 
aqueous Foundations,"  says,  referring  to  dippers, 

"Such  dredges  are  more  simple  in  construction  than  elevator  (ladder) 
dredges  ....  and  are  consequently  easier  and  cheaper  to  keep  in 
repair,"  and  again,  that  many  dippers  have  "sufficient  power  to  dig 
hardpan,  boulders  and  very  soft  shale  rock,"  and  yet  again:  "The 
ladder  dredge,  being  so  much  more  expensive  to  operate  and  keep  in 
repair  than  clam-shell  or  dipper  machines,  is  seldom  used  in  the  United 
States." 

On  the  other  hand,  some  engineers  have  criticised  the 
American  for  his  disregard  of  the  advantages  and  efficiency 
of  the  ladder  type,  and  have  excused  this  shortcoming  by 
the  explanation  that,  in  the  development  of  this  new 
country,  dredging  operations  were  secondary  to  larger 
and  more  important  improvements,  and,  consequently, 
the  small  appropriations  and  the  dearth  of  available  funds 
resulted  in  the  construction  of  small  dredges  of  the  cheapest 
possible  type,  and  that,  later,  when  dredging  operations 
assumed  larger  proportions,  the  manufacturers  built  the 
more  powerful  machines  along  the  lines  of  the  earlier  plant. 
Recently,  however,  the  development  of  the  ladder  dredge 
has  been  fairly  rapid,  both  in  the  gold  mining  industry 
of  the  western  States  and  in  the  excavation  of  commercial 
sand  and  gravel  in  the  east.  Furthermore,  the  small 
French  elevator  machines,  rebuilt  and  operated  by  the 
United  States  Government  on  the  Panama  Canal,  excited 
favorable  comment  by  their  good  work  there.  It  would 
appear,  therefore,  that  the  criticism  of  dredging  bigotry 
may  justly  be  applied  to  both  Europe  and  America;  to 
Europe  for  her  failure  to  recognize  .the  merits  of  the  dipper 
and,  to  a  less  degree  of  the  grapple,  and  to  America,  for 
our  obstinacy  in  refusing  to  consider  the  advantages  of  the 
elevator  dredge.  Undoubtedly,  there  are  specific  purposes 
and  peculiar  conditions  for  which  each  type  is  best  adapted, 
and  the  earlier  the  recognition  of  this  fact,  the  better  for 
the  dredging  world. 

Hydraulic  machines  are  used  extensively  both  here  and 
abroad.  Although  first  employed  in  France  in  1867,  they 


110  DREDGING  ENGINEERING 

have  reached  a  high  state  of  development  in  this  country, 
and  Fowler  even  goes  so  far  as  to  claim  that  "The  suction 
dredge  is  essentially  an  American  tool."  It  is  a  particularly 
efficient  instrument  in  that  it  not  only  dredges  the  material 
but  also  delivers  it  to  the  place  and  at  the  elevation  desired 
with  one  operation.  The  cost  of  the  dredge  per  capacity 
is  less  than  that  of  other  machines  and,  moreover,  it  is 
practically  impossible  to  equal  in  any  other  type  the 
enormous  capacity  of  some  hydraulic  dredges  in  service. 
While  it  can  be  used  in  all  classes  of  material  short  of  solid 
rock,  it  is  most  efficient  in  handling  homogeneous  material 
that  is  not  excessively  hard,  such  as  silt,  mud,  sand,  clay 
and  gravel,  reasonably  free  of  obstructions.  Mud  and  fine 
sand  are  regarded  as  the  best  hydraulic  materials.  The 
pump  can  handle  a  greater  percentage  of  solids  when 
working  in  clay,  but  difficulty  is  experienced  in  agitating 
and  cutting  it  in  sufficient  quantity  to  maintain  the  feed 
at  the  maximum  capacity.  The  length  of  pipe  line,  as  a 
controlling  factor  in  dredging  economy,  varies  with  the 
nature  of  the  pumpings.  For  heavy  material,  such  as 
coarse  sand,  gravel  and  clay,  the  maximum  economic 
length  of  line  is  less  than  for  light  materials  easily  carried 
in  suspension,  such  as  mud  and  fine  sand.  The  greater 
tendency  of  the  heavier  pumpings  toward  rapid  deposit, 
and  the  consequent  decrease  in  the  percentage  of  solid 
matter  handled,  coupled  with  increased  pipe  friction,  causes 
outputs  on  long  lines  smaller  than  for  the  light  material. 
The  grapple  dredge  is  at  its  best  in  mud  and  stiff  mud. 
Swinging  a  soft-digging  bucket  of  large  capacity  and  fitted 
with  side-boards,  it  can  maintain  an  output  in  such  material 
in  excess  of  dippers  and  ladders  of  equal  expense.  In 
harder  material,  however,  as  sand,  gravel  and  clay,  the 
hard-digging  bucket  of  smaller  size  must  be  substituted, 
and  moreover,  the  weight  of  the  bucket,  decreased  by  the 
lifting  tendency  of  the  closing  wire  is  often  insufficient  to 
obtain  the  penetration  necessary  to  obtain  a  full  load.  In 
other  words,  the  closing  bucket  scrapes  over  the  resistant 
bottom  making  a  shallow  bite  and-  only  partially  filling 


PRELIMINARY  ENGINEERING  111 

itself  (See  Chapter  II) .  In  soils  of  this  kind,  the  dipper  and 
the  ladder  will  give  better  results  than  the  grapple.  The 
dipper  is  particularly  well  adapted  to  hard  digging  and  to 
the  removal  of  material  containing  a  large  amount  of 
obstructions.  It  can  readily  handle  stone  and  boulders 
and  even  soft  rock.  The  grapple  may  be  equipped  to 
dredge  stone  and  boulders  by  the  use  of  a  special  open- 
tined  stone  grapple  in  place  of  the  usual  clam-shell  bucket, 
but  the  dipper  is  preferable.  In  fact,  when  the  material, 
whether  the  bulk  be  soft  or  compact,  is  full  of  obstruction 
such  as  stone,  boulders,  snags,  piling  or  subaqueous  struc- 
tures, the  dipper  is  the  most  efficient  machine  in  use,  but 
in  soft,  homogeneous  material,  the  grapple  is  better. 
Especially  is  the  pin-up  dipper  ill  suited  to  soft  digging, 
because  such  soil  offers  insufficient  resistance  to  the  penetra- 
tion of  the  spuds,  upon  which  the  machine  depends  in  the 
pinned-up  condition.  In  clay  or  in  hard  sandy  soils,  the 
efficiency  of  the  grab  bucket  may  be  somewhat  increased 
by  the  addition  of  teeth  or  tines  to  its  cutting  edges,  but 
the  American  tendency  is  rather  to  depend  upon  the  great 
weight  of  the  bucket.  The  orange  peel  bucket  is  seldom 
used  on  large  operations  but  is  often  employed  on  small 
grapples  in  casting  over  work  and  in  construction  work 
where  there  is  more  or  less  loose  rock  or  obstruction  to 
pick  up,  or  in  the  interior  of  cofferdams  and  caissons. 
A  consideration  appurtenant  to  the  use  of  bucket  dredges 
working  in  clay  is  the  difficulty  often  experienced  in  dump- 
ing the  scows.  The  clay  becomes  compacted  in  the  pockets 
into  a  sticky  mass  arched  across  the  door  openings  and  will 
at  times  remain  in  the  scows  for  hours  with  the  doors  open. 
Jetting  may  be  resorted  to  in  such  instances. 

The  ladder  dredge  working  in  free  sand  experiences 
some  difficulty  in  maintaining  full  buckets,  due  to  the  agi- 
tation of  the  sand  and  the  consequent  loading  of  the  buckets 
with  a  mixture  of  sand  and  water  with  sand  in  suspension. 
In  sand  and  mud,  therefore  the  buckets  are  run  at  a 
low  velocity,  or  the  same  effect  is  obtained  by  spacing 
them  further  apart  by  the  interpositon  of  a  link 


112 


DREDGING  ENGINEERING 


between  each  pair  of  buckets.  In  refractory  material, 
the  bucket  chain  is  speeded  up  to  get  the  benefit  of  impact 
or  else  the  buckets  are  spaced  more  closely. 

The  Table  page  112,  is  an  effort  to  group  the  four 
dredge  types  in  the  order  of  their  preference  for  each  of  the 
classes  of  material  usually  encountered  in  dredging  opera- 
tions. Beginning  at  the  left  with  "Mud, "  the  various 
soils  are  listed  in  the  order  of  their  difficulty  of  removal 
from  the  dredging  standpoint.  The  first  named  dredge 
under  each  heading  is  the  type  best  adapted  to  that  kind 
of  soil,  the  second  the  next  most  desirable,  and  so  on.  It 
must  be  borne  in  mind  that  this  selection  is  made  purely 
upon  the  basis  of  the  nature  of  the  material,  assuming 
that  all  other  conditions  are  ideal  for  the  operation  of  each 
dredge  in  the  material  named,  and  that  the  soils  are  homo- 
geneous and  free  of  obstruction. 


1.  Mud. 


2.   Mud  and  sand.  3.  Coarse  sand.       4.  Fine  sand. 


5.  Sand  and 
gravel. 


Hydraulic 
Grapple 
Dipper 
Ladder 

Hydraulic 
Grapple 
Dipper 
Ladder 

Hydraulic 
Ladder 
Dipper 
Grapple 

Hydraulic 
Dipper 
Ladder 
Grapple 

Dipper 
Ladder 
Hydraulic 
Grapple 

6.  Gravel. 


7.  Stiff  clay. 


8.  Indurated  clay  or 
hard  pan. 


Ladder 
Dipper 
Grapple 
Hydraulic 

Dipper 
Ladder 
Hydraulic 
Grapple 

Ladder 
Dipper 

In  regard  to  the  character  of  the  bottom  soil,  therefore, 
the  hydraulic  and  dipper  dredges  hold  the  high  scores,  but 
upon  a  consideration  of  the  other  factors,  it  will  be  found 
that  the  grapple  type  will  assume  a  more  important  role. 

b.  In  some  localities,  the  War  Department  Engineers 
have  set  aside  certain  areas,  called  free  dumps,  over  which 
it  is  permitted  that  dredged  material  be  deposited  from 
scows.  Disposal  in  this  manner  is  regulated  by  the  War 
Department  and  supervised  by  inspectors  in  their  employ, 
who  are  stationed  on  the  tow  boats  taking  the  scows  to 
and  from  the  dump,  and  whose  salaries  are  paid  by  the 
dredging  contractor.  Buoys  and  lights  on  the  dumps  must 


PRELIMINARY  ENGINEERING  113 

also  be  maintained  by  and  at  the  expense  of  the  contractor. 
The  cost  of  such  deposit  is  reduced  to  the  per  cubic  yard 
basis  and  is  a  function  of  the  number  of  yards  handled  and 
the  cost  of  the  scows,  tug  boats,  crews,  inspection  and  buoy 
maintenance  for  any  day,  involving  the  number  of  round 
trips  made  in  one  day  and  the  number  and  size  of  scows 
in  each  tow.  Tidal  and  seasonal  weather  conditions  may 
have  an  important  bearing  and  must  be  discounted.  In 
bad  weather,  dredging  operations  may  even  be  entirely 
suspended  for  a  time  because  of  the  inability  of  the  tugs  to 
make  the  tow.  In  some  harbors,  located  on  or  near  the 
seaboard,  material  is  towed  to  sea  and  dumped.  The 
total  cost' of  the  dredging  then  is  the  sum  of  the  cost  of  the 
actual  operation  of  digging  by  bucket  machines  and  loading 
the  scows,  plus  the  cost  of  transportation  to  the  dump, 
including,  of  course,  the  use  of  the  plant.  Should  no  free 
dump  be  available,  or  the  towing  distance  be  too  great, 
the  alternative  is  the  hydraulic  rehandling  of  the  scowed 
material  into  an  impounding  basin. 

(c)  The  stationary  hydraulic  dredge  requires  a  nearby 
impounding  basin  or  a  place  to  receive  and  contain  the 
pumpings,  and  the  cost  and  other  features  of  this  indis- 
pensable adjunct  will  very  often  decide  for  or  against  the 
use  of  suction  machines.  Provided,  therefore,  that  the 
material  to  be  dredged  is  such  as  to  permit  of  hydraulic 
removal,  a  study  of  the  topography  of  the  adjacent  terrain 
and  of  the  hydrography  of  the  abutting  shore  line  is  made 
for  the  purpose  of  determining  its  availability  for  impound- 
ing uses  and  estimating  the  expenditures  necessary  for 
the  acquisition  of  the  property  or  of  the  filling  rights  and 
for  the  construction  of  the  basin.  The  ideal  basin  will 
have  the  following  features: 

1 .  A  Large  Capacity  for  a  Small  Construction  Cost. — The 
principal  item  of  expense  is  that  of  the  enclosing  or  retain- 
ing structure,  the  bank  or  dike,  and  it  is  obviously  an  eco- 
nomic advantage  that  the  initial  and  maintenance  banking 
cost  per  cubic  yard  of  basin  .capacity  be  small. 

2.  Maximum  Increase  in  Property  Value  Accruing  from 


114  DREDGING  ENGINEERING 

the  Fill.— Whether  the  filling  effects  the  rehabilitation  of 
swamps  and  marshes  or  the  reclamation  of  submerged 
waterfront  areas,  it  naturally  enhances  the  value  of  the 
property.  For  this  reason,  the  basin  privilege  may  often 
be  obtained  from  land  owners  without  charge,  obviating 
the  necessity  of  purchase.  In  some  cases  the  positions  of 
creditor  and  debtor  may  even  be  reversed.  Should  the 
basin  project  beyond  the  shore  line,  questions  may  arise 
as  to  riparian  rights,  the  slip  rights  of  adjoining  owners, 
the  location  of  bulkhead  lines  established  by  local  or  govern- 
ment authorities  and  the  granting  of  license  to  build  by 
those  same  departments. 

3.  A  short  pipe  line  from  dredge  to  basin. 

4.  Easy   Accessibility. — -By   this   is   meant  a  minimum 
cost  of  pipe  line  communication  from  the  pontoon  line  to 
the  basin,  or,  in  other  words,  a  dearth  of  expensive  pipe 
trestles  and  of  obstruction  to  laying  the  line. 

5.  A  Low  Final  Grade. — High  elevations  of  discharge  pipe 
mean  increased  head  on  the  pump  and  reduced  output. 

6.  Natural  Disposal  of  the  Effluent. — The  great  quantity 
of  water  accompanying  the  soilds  must  be  returned,  in 
the  great  majority  of  cases,  to  the  source  from  which  it  was 
pumped.     At  the  same  time,  it  is  desirable  that  the  waste- 
weir  or  sluice  be  remote  from  the  pipe  discharge  in  order 
to  give  opportunity  for  the  precipitation  or  impounding 
of  the  solid  matter   carried  in  suspension  in  the  water. 
Should  such  location  of  the  sluice-box  preclude  the  natural 
flow  of  the  effluent  from  sluice  to  source,  an  artificial  canal 
or  flume  becomes  necessary. 

In  making  an  economic  choice  between  bucket  and  hy- 
draulic plant  for  a  dredging  operation  in  so  far  as  the  im- 
pounding basin  is  concerned,  the  problem  may  be  stated 
briefly  thus:  Does  the  accrued  value  of  the  basin  property 
plus  the  saving  effected  by  direct  hydraulic  removal  in 
lieu  of  bucket  dredging  and  scowing  away  justify  the  ex- 
pense of  the  construction  of  the  empty  basin,  comprising 
acquisition  of  property  and  the  initial  and  maintenance 
cost  of  banks,  sluice,  drainage  canal  and  pipe  lines? 


PRELIMINARY  ENGINEERING  115 

The  quite  common  use  of  impounding  basins  for  the 
disposal  of  bucket-dredged  material  presents  a  different 
problem.  Here  the  contents  of  the  scows  are  dumped  to  a 
pump  located  at  the  basin  and  rehandled  hydraulically  into 
it,  in  which  case,  the  rehandling  and  basin  expense  are 
appurtenant  to  the  cost  of  the  bucket  dredging. 

(d)  The  first  feature  of  the  fourth  factor,  viz.  the  kind 
of  plant  available,  is  self-evident.     Naturally  the  selection 
of  plant  must  be  made  from  that  which  can  be  brought  to 
the  job  without  prohibitive  transportation  expense.     The 
project  may  clearly  be  a  bucket  job  and  yet,  because  of  the 
impossibility  of  obtaining  enough  bucket  plant  to  complete 
it  on  time,  the  necessity  may  arise  for  the  assistance  of 
available  hydraulic  machines. 

Dredges  of  the  same  type  often  have  distinguishing 
features  and  characteristics  peculiar  to  themselves.  This 
grapple  or  dipper  is  slow  and  that  one  fast;  or  this  suction 
dredge  is  no  good  on  the  long  pipe  lines;  or  this  dredge  is 
underpowered,  or  a  poor  steamer,  or  a  coal  glutton;  this 
machine  has  a  better  captain,  or  better  quarters  for  the  men 
and  therefore  is  better  able  to  hold  her  crew;  or  this  pump 
has  a  better  cutter  head  and  this  one  has  too  light  a  ladder 
to  hold  the  cutter  down  in  hard  stuff,  etc.,  etc.  Naturally 
those  units  are  desired  which  will  give  economically  the 
greatest  output. 

(e)  As  to  space  limitations  and  interference  with  traffic : 
The  size  and  form  of  the  area  to  be  dredged  and  the  pres- 
ence of  traffic  may  be  an  important  consideration  in  the 
selection  of  type  and  even  in  the  choice  between  dredges  of 
the  same  type.     Hydraulic  dredges  with  radial  feed  about 
the  stern  spud  require  considerable  space  for  efficient  opera- 
tion because  of  their  pontoon  lines,  possible  pipe  trestles, 
the  great  width  of  cut  and  the  presence  of  swinging  wires  on 
both  sides.     Those  having  swinging  ladders  can  work  in 
more  confined  areas,  due  to  the  narrower  width  of  cut  and  the 
absence  of  swinging  wires.     Bucket  dredges  must  have 
enough  room  for  towing  scows  to  and  from  the  machines 
and  for  handling  the  scows  at  the  dredge.     It  will  be  remem- 


116  DREDGING  ENGINEERING 

bered,  that,  at  the  commencement  of  the  loading  of  a  scow, 
almost  its  entire  length  extends  forward  beyond  the  bow  of 
the  dredge  and  also  that  dredges  are  usually  rigged  for  right- 
handed  digging,  i.e.  with  the  scow  on  the  starboard  side. 
The  head  of  a  narrow  slip,  therefore,  may  be  practically  in- 
accessible to  a  clam  shell  dredge  and  its  accompanying  scow, 
or  may  necessitate  the  use  of  a  grapple  and  scows  of  smaller 
capacity.  Limited  space  or  traffic  congestion  may  give  rise 
to  the  necessity  for  the  use  of  bucket  dredges  having  suffi- 
cient spud  equipment  to  obviate  the  necessity  for  wires  and 
anchors.  In  throwing  up  banks  or  in  dredging  drainage 
and  irrigation  ditches,  it  is  desirable  to  have  dredges  of 
small  beam,  where  the  conditions  are  such  that  the  narrow- 
est possible  cut  is  all  that  is  required.  Small  grapples  and 
dippers  swinging  %  to  1J^  yd.  buckets  on  long  booms  are 
generally  employed  for  such  work  and  are  called  banking 
machines.  Some  such  are  equipped  with  bank  spuds  in 
lieu  of  the  usual  type.  Bank  spuds  are  inclined  away  from 
the  dredge  and  bear  upon  a  small  grillage  resting  on  the 
top  of  the  bank,  thus  maintaining  the  dredge  upon  an 
even  keel  in  resisting  the  listing  moment  of  the  long  boom. 

(/)  Both  the  existing  depth  and  the  depth  to  be  dredged 
are  considerations  in  the  plant  selection.  Since  the  scows 
being  loaded  by  grapples  and  dippers  extend  forward 
beyond  the  bow  of  the  dredge  and  in  advance  of  the  dredged 
cut,  the  before-dredging  depth  of  water  must  be  sufficient 
to  float  the  loaded  scows  in  order  to  permit  the  dredge  to 
work.  This  flotation  depth  is  usually  about  10  to  12  feet. 
If  the  natural  water  is  too  shoal,  therefore,  for  loading 
scows  at  all  stages  of  the  tide,  a  channel  called  a  "  pilot 
cut"  must  be  dredged  for  them.  The  material  from  such 
preliminary  work,  obviously,  cannot  be  scowed  away,  but 
must  simply  be  "cast  over"  to  one  side,  forming  a  spoil 
bank,  which  is  removed  later  in  scows.  A  dredge  may  do 
its  own  pilot  cutting,  but  a  more  efficient  method  is  the  use 
of  smaller,  less  expensive,  long-boom  machines. 

Hydraulic  dredges  require  flotation  for  a  sufficient  length 
of  their  pontoon  lines  to  allow  the  necessary  swinging 


PRELIMINARY  ENGINEERING  117 

freedom  and  advance  motion.  Three  or  four  feet  will  float 
the  ordinary  pontoon  for  a  20  inch  machine.  The  draught 
of  the  dredge  itself  may  be  a  factor.  Dredges  must  float 
to  operate.  The  process  of  making  flotation  depth  for 
their  own  hulls  is  termed  "  digging  themselves  in."  Con- 
siderations of  this  nature,  more  particularly  in  ditching 
and  banking  operations,  may  establish  a  preference  for 
light  draught  machines. 

The  final  depth  specified  and  the  vertical  distance 
between  the  original  and  final  depths,  which  is  called  the 
" depth  of  cut"  or  " height  of  bank,"  may  influence  the 
choice  of  plant.  A  grapple  machine  is  not  limited  to  any 
depth  except  by  the  length  of  the  bucket  wires  and  the 
wire  capacity  of  the  drums.  The  dipper  is  limited  first, 
by  the  length  of  dipper  stick,  and,  second,  by  her  power 
to  develop  the  necessary  forward  thrust  of  the  dipper, 
which  decreases  as  the  depth  increases.  A  dipper  will 
seldom  be  economical  in  depths  greater  than  35  to  40  feet. 
The  maximum  depth  of  which  the  hydraulic  is  capable  is  a 
function  of  the  length  of  ladder  and  the  design  of  the  ladder 
hinge.  For  all  around  work,  a  20  inch  or  larger  machine 
should  be  designed  to  dredge  to  38-42  feet  below  the  water 
surface,  although  the  requirements  will  vary  with  the 
locality  and  purpose  for  which  the  machine  is  built.  The 
depth  capabilities  of  the  elevator  will  be  controlled  by  the 
length  of  ladder  and  the  engine  power.  The  deeper  the 
water,  the  greater  the  work  done  in  the  same  time  because 
of  the  increased  lift. 

Hydraulic  and  ladder  machines  are  better  adapted  to 
small  "depths  of  cut,"  or  "height  of  bank"  than  are 
grapples  and  dippers. 

(g)  Finally,  time  may  be  a  most  pertinent  element  of 
the  problem.  An  emergency  job  of  some  magnitude  may 
require  that  the  considerations  of  economy  and  efficiency 
be  temporarily  waived  to  a  certain  extent  in  order  that  the 
complete  removal  of  a  given  yardage  be  accomplished  in 
a  certain  time.  The  rate  of  the  dredging  may  be  the 
governor  of  an  entire  project. 


118  DREDGING  ENGINEERING 

Construction  progress  on  piers,  wharves,  shipways, 
dry  docks  or  ship  launchings  may  hinge  upon  the  dredging. 
It  is  hardly  necessary  to  enumerate  the  multifarious  ways 
in  which  the  completion  of  dredging  by  a  given  date  can 
and  does  acquire  great  importance.  The  problem  then  is  to 
assemble  a  plant  that  will  do  the  work  in  a  hurry.  It  may 
involve  the  expense  of  the  construction  of  an  impounding 
basin  to  receive  rehandled  material  from  bucket  dredges, 
even  though  scowing  away  be  more  economical.  It  may 
mean  use  of  bucket  dredges  in  lieu  of  less  expensive  hydrau- 
lic machines,  because  of  the  ability  to  work  several  bucket 
machines  where  but  one  pump  could  operate.  It  may  mean 
the  employment  of  pilot  machines  which  would  be  un- 
necessary if  there  were  sufficient  time  to  permit  the  dredg- 
ing to  pursue  its  economic  course.  Direct  pumping  may 
be  necessary  in  order  to  get  large  output,  even  though  the 
cost  of  the  basin  be  otherwise  prohibitive. 

Plans,  Specifications  and  Contracts. — There  are  several 
principal  bases  upon  any  one  of  which  a  dredging  contract 
is  written,  the  preference  depending  in  great  measure 
upon  the  definiteness  and  degree  of  certainty  of  the  in- 
formation as  to  the  extent,  nature  and  working  conditions 
of  the  proposed  project.  The  completeness  and  text  of 
the  specifications  and  contract  will  naturally  depend  upon 
the  basis  adopted.  The  choice  usually  lies  between 

1  The  unit  price  basis,  by  which  the  contractor  is  paid 
a  certain  price,  fixed  by  the  agreement,  for  each  cubic  yard 
of  material  dredged,  measured  either  in  the  scows  or  in 
place  in  the  cut. 

2.  The  cost  plus  percentage  basis,  by  which  the  con- 
tractor is  paid  the  actual  cost  of  the  work  plus  a  fixed 
percentage  of  that  cost. 

3.  The  cost  plus  fee  basis,  by  which  the  contractor  is 
paid  the  actual  cost  of  the  work,  plus  a  definite  sum  of 
money  or  fee,  fixed  by  the  contract. 

4.  The  lease  or  rental  basis,  by  which  the  contractor  is 
paid  a  fixed  rental  for  his  plant  for  the  period  of  its  use,  in 
full  compensation  for  the  performance  of  the  work  specified. 


PRELIMINARY  ENGINEERING  119 

5.  The  cost  plus  rental  basis,  by  which  the  contractor  is 
paid  the  actual  cost  of  the  work  plus  a  fixed  periodical 
rental  in  compensation  for  the  use  of  his  plant  and  services. 

6.  The  lump  sum  basis,  by  which  a  sum  of  money  is 
mutually  agreed  upon  as  full  compensation  for  the  entire 
project  complete,  and  is  usually  paid  out  to  the  contractor 
in  monthly  installments  in  amounts  varying  with  the  esti- 
mated quantity  of  material  removed  during  each  month. 

The  first  method  is  the  most  common  and  is  in  general 
favor  with  the  owners  of  parties  footing  the  bills.  They 
have  the  satisfaction  of  knowing  in  advance  within  narrow 
limits  the  ultimate  cost  of  the  work  to  them.  Where  an 
abnormal .  element  of  uncertainty  exists,  however,  as  to 
the  extent  of  the  work,  the  nature  of  the  material  or  the 
condition  of  the  labor  and  supplies  markets,  it  is  not  only 
manifestly  unfair  to  the  contractor  to  pin  him  down  to  a 
definite  yardage  price,  but  it  is  very  probable  that  the  work 
will  cost  the  owner  more  money  than  would  be  required 
under  some  other  form  of  contract.  Under  such  conditions, 
the  contractor  will  usually  bid  high  to  cover  possible 
contingencies,  which  are  thus  paid  for  whether  or  not  they 
materialize,  and  the  owner  had  done  better  to  accept  a 
basis  of  payment  such  as  numbers  3  or  5  above.  The  sixth 
basis  is  seldom  employed  in  dredging,  because  of  the  dis- 
crepancies between  estimated  and  actual  yardages. 

Dredging  is  no  exception  to  the  rule,  prescribed  both 
by  economy  and  ethics,  that  the  information  furnished 
bidding  contractors  be  as  complete  as  it  lies  within  the 
power  of  the  engineer  to  provide.  Rather  is  it  a  branch  of 
contracting,  to  which,  because  of  its  inherent  uncertainty, 
this  is  particularly  applicable. 

It  is  not  necessary  to  review  here  the  general  clauses, 
indigenous  to  all  dredging  contracts.  An  outline  of  the 
specific  information  that  should  appear  in  plans  and  specifi- 
cations accompanying  invitations  to  bid  a  unit  price  follows : 

1.  A  map  of  the  waterway  showing  the  area  to  be  dredged 
and  the  places  of  deposit;  the  existing  depths  of  water  over 
this  area  and  in  adjacent  reaches,  recorded  from  recent 


120  DREDGING  ENGINEERING 

soundings;  the   topography  of  the  places  of  deposit  if  in 
the  dry;  and  finally  the  locations  of  borings. 

2.  A  description  of  the  principal  characteristics  of  the 
waterway,  as  to  tidal  range,  freshets,  ice,  etc. 

3.  A  clear  exposition  of  the  work  to  be  done:     the  length, 
width  and  depth  of  the  proposed  channel,  the  method  of 
disposal  and  the  approximate  quantity  of  material  to  be 
removed,  place  or  scow  measurement. 

4.  A  definition  of  the  pay  quantities,  the  method  of 
measurement  and,  if  necessary,  the  ratio  for  the  conversion 
of  scow  to  place  measurement  and  vice  versa. 

5.  The  limits  of  the  cross  section  of  the  proposed  channel, 
the  side  slopes  and  the  overdepth  allowance,   and  the 
amount  of  money  to  be  deducted  for  each  yard  removed 
from  outside  these  limits. 

6.  The  character  of  the  material  to  be  dredged  as  deter- 
mined by  borings,  samples  of  which  should  be  available 
for  examination ;  whether  original  bottom  or  sedimentation ; 
and  a  definite  limit  to  the  difficulty  of  the  material  to  be 
removed  without  additional  compensation. 

7.  A  clear  statement  of  the  basis  of  acceptance  of  dredged 
sections. 

8.  A  definite  line  of  demarkation  between  the  duties  of 
the  contractor  and  the  engineer  as  to  ranges,  buoys  and  field 
work  generally. 

9.  Appurtenant  items  of  expense  to  the  contractor,  such 
as  inspection,  dump  and  basin  maintenance,  etc. 

10.  Responsibility  for  dikes,  sluiceways  and  pipe  lines. 

11.  Limits  for  starting  and  completion,  if  desired,  and 
the  order  of  work. 

Obviously,  with  the  other  forms  of  contract,  some  of 
the  above  data  is  superfluous,  but  on  the  other  hand,  cer- 
tain additional  information  is  necessary.  In  those  con- 
tracts involving  a  determination  of  the  cost  of  the  work, 
the  make-up  of  that  cost  must  be  clearly  defined,  and  in 
those  having  the  rental  feature,  an  understanding  is 
necessary  as  to  the  plant  leased,  towage  charges,  wages 
of  crews,  fuel  and  supplies,  and  precisely  what  lost  time  is 


PRELIMINARY  ENGINEERING  121 

chargeable  to  the  contractor  and  what  to  the  lessee.  The 
latter  may  be  protected  by  a  specified  guarantee  of  output 
for  each  dredge.  For  a  portion  of  the  original  dredging 
work  at  the  Hog  Island  Shipyard,  hydraulic  dredges  were 
leased  with  the  understanding  that  reductions  in  output 
below  6000  yards  per  24  hour  day  through  a  pipe  line  not 
longer  than  3500  feet  would  entail  proportionate  reductions 
in  rental.  This  guarantee,  however,  was  subsequently 
waived,  owing  to  the  material  being  more  difficult  than  the 
mud  and  sand  specified. 

Scheduling. — In  order  to  predetermine  the  size  of  the 
plant  and  the  most  advantageous  disposition  of  the  units 
thereof,  that  the  project  may  be  completed  within  a  given 
time,  a  dredging  schedule  is  drawn  up.  Even  though  the 
uncertainities  of  dredging  render  it  impossible  that  the 
schedule  embody  the  same  degree  of  precision  and  expect- 
ancy ascribed  to  those  of  other  classes  of  work,  yet  it  is  very 
necessary  to  the  successul  prognostication  of  the  results 
and  very  useful  in  the  coordination  of  the  units  engaged. 
The  schedule  is  a  program  of  operation,  showing  the  num- 
ber and  names  of  the  machines  employed  upon  each  division 
or  zone  of  the  work,  the  length  of  time  from  date  to  date 
that  each  item  of  plant  remains  thereon,  the  yardage  as- 
sumed to  be  the  daily  average  output  of  which  each  dredge 
is  capable,  and  the  total  yardage  to  be  removed  from  each 
zone.  It  is  an  effort  so  to  locate  each  dredge  as  to  provide 
conditions  to  which  that  machine  is  best  adapted,  and  to 
coordinate  to  the  fullest  extent  the  project  as  a  whole  by 
intelligent  anticipation  of  the  growth  of  space  and  depth 
limitations  affecting  the  relative  location  of  the  dredges. 
The  success  of  the  schedule  will  depend  in  large  measure 
upon  the  correctness  of  the  assumed  daily  average  output 
of  each  machine,  which  must  be  low  enough  to  take  care  of 
a  reasonable  amount  of  lay  time  due  to  repairs,  etc.,  and 
upon  the  ability  of  each  dredge  to  endure  through  the  life 
of  the  job  without  serious  accident.  The  fundamental 
weakness  of  a  dredging  schedule  is  that  the  output  is 
dependent  upon  a  small  number  of  units  of  large  capacity, 


122 


DREDGING  ENGINEERING 


so  that  a  loss  of  any  one  of  them  entails  a  large  percentage 
of  reduction  in  the  working  plant. 

As  always  in  problems  of  this  character,  the  best  method 
of  attack  is  first  to  define  specifically  the  ideal  schedule 
and  then  so  to  modify  it  as  to  incorporate  to  the  best  ad- 
vantage and  with  minimum  deviation  from  the  ideal  all 
the  practical  considerations  and  circumstances  of  operation 
that  must  be  taken  care  of.  A  form  similar  to  that  of 
Fig.  38,  page  122,  is  very  helpful.  The  entire  area  to  be 


DREDGING  SCHEDULE 

/.one 

Est.  Yardage 

Dredge 

Daily 
Av'ge 

\N  ork- 
& 

Quota 

April 

May 

June 

July 

Aug. 

Bucket 

Hydr'c 

1 

5  22 

7    1 

5  22 

A 

300,000 

— 

No.l 

4,000 

25 

100,000 

4- 

I 

No.  2 

3,000 

25 

75,000 

No.  3 

2,500 

50 

125,000 

B 

200,000 

— 

No.l 

25 

100,000 

No.  2 

34 

100,000 

C 

75rOOO 

No.  3 

30 

75,000 

D 

5:0,000 

No.  4 

5,000 

60 

300,000 

No.  5 

6,001 

34 

200,000 

E 

etc. 

etc. 

Ebb  a 

udler 

No.6 

12,000 

C  75,  000 

Total 

! 

Date  —                                                                                              Approved  — 

FIG.  38. — Convenient  form  of  schedule. 

dredged  is  first  divided  into  a  number  of  blocks  or  zones  and 
the  yardage  in  each  is  computed.  The  size  and  boundaries 
of  the  zones  will  vary  with  the  nature  of  the  job,  but,  where 
possible,  it  is  convenient  to  have  them  defined  by  such 
tangible  limits  as  may  exist,  by  the  ranges  establishing  the 
dredge  cuts  and  by  the  dividing  lines  between  bucket  and 
hydraulic  areas.  The  location  of  shore  pipe  lines  and  pipe 
trestles  is  a  factor  in  the  hydraulic  zone  layout,  since  the 
location  and  field  of  operation  of  each  pump  dredge  is 
dependent  thereon.  By  making  the  horizontal  lines  rep- 
resenting the  periods  of  operation  in  the  zones  a  separate 
color  for  each  machine,  the  itinerary  of  each  dredge  can 
readily  be  followed  down  the  page. 


PRELIMINARY  ENGINEERING  123 

Estimating  the  Cost. — The  complete  cost  of  a  dredging 
project  from  the  time  of  its  inception  comprises  five 
principal  items,  as  follows : 

1.  Cost  of  preliminary  engineering. 

2.  Cost  of  permits,  licenses  and  acquisition  of  property  or  filling  rights 
for  impounding  basins. 

3.  Cost  of  preliminary  structures. 

4.  Cost  of  operation  of  the  dredging  plant. 

5.  Cost  of  maintenance  of  the  dredged  depths. 

The  first  consists  of  the  salaries  of  engineers,  office  force 
and  field  parties  engaged  in  making  the  preliminary  sur- 
veys, exploring  the  site  and  preparing  plans,  specifications, 
estimates r  schedules  and  contracts;  the  cost  of  their  equip- 
ment and  supplies;  the  cost  of  labor,  equipment  and  sup- 
plies used  in  making  borings  and  probings;  overhead, 
comprising  salaries  of  executives,  legal  advisory  talent, 
office  rent,  insurance  and  incidentals. 

The  second  is  self-explanatory. 

The  third  includes  the  cost  of  construction  of  dikes, 
sluice-ways,  drainage  ditches,  shore  pipe  lines,  involving 
excavation,  cribbing,  trestles  and  pipe  laying;  the  cost 
of  construction  plant,  repairs,  depreciation  and  sinking 
fund;  cost  of  materials,  labor,  supplies  and  transportation; 
cost  of  field  office,  superintendence  and  general  overhead. 

The  fourth  item  is  the  cost  of  the  actual  removal  of  the 
material  and  obviously  is  the  largest  part  of  the  cost  of  the 
job.  Only  when  the  engineer  is  directly  operating  the 
dredging  plant  will  he  be  concerned  in  detail  with  all  the 
numerous  components  making  up  this  item.  His  point 
of  view  is  influenced  by  the  nature  of  the  contract.  If  the 
work  is  to  be  paid  for  upon  a  per  cubic  yard  basis,  he  will 
add  to  that  unit  price,  the  cost  of  operating  items  not 
covered  thereby,  as  dump  and  basin  maintenance,  super- 
intendance  and  inspection,  hydrography  and  overhead, 
after  reducing  them  also  to  a  yardage  basis.  If  the  con- 
tract be  one  of  lease,  he  figures  for  each  plant  unit  the  cost 
per  diem  of  the  bare  boat,  the  crew,  grub,  fuel  and  such 
items  as  are  not  included  in  the  bare  boat  rental,  after 


124  DREDGING  ENGINEERING 

which,  from  an  assumed  daily  output,  he  prognosticates 
the  length  of  duration  of  the  lease.  To  this,  of  course, 
must  again  be  added  the  items  of  dump  and  basin  main- 
tenance, etc.,  as  above. 

The  components,  in  detail,  of  the  cost  of  operation  are 
as  follows: — • 

(a)  Plant Transportation 

Repairs 
Depreciation 
Sinking  Fund 
Insurance 

(b)  Labor. .  * Dredge  crews 

Tug  crews 

Miscellaneous  plant  crews 
Pipe  line  men 
Insurance 

(c)  Supplies Fuel,  oil  and  waste 

Food  and  miscellaneous  supplies 

Furnishings 

Tools 

(d)  Dump  and  Basin    Buoys 

Maintenance   . .     Lights 

Dike  patrol 

Dike  and  sluice  repairs 

(e)  Superintendence  and  Inspection. 

(/)    Hydrographer's  Force,  Equipment  and  Supplies, 
(g)  General  Overhead,  both  Contractors  and  Engineers. 

The  fifth  item,  maintenance  of  dredged  depths  is  a 
subsequent  expenditure,  but  may  be  a  factor  in  the  selec- 
tion of  channel  location.  It  involves  not  only  the  cost  of 
redredging,  but  also  the  maintenance  of  controlling 
structures. 


CHAPTER   X 
PRELIMINARY  CONSTRUCTION 

Dikes  for  River  Control. — Dredging  and  the  maintenance 
of  dredged  depths  is  intimately  related  to  the  subject  of 
the  characteristics  and  regulation  of  rivers.  This,  however, 
is  much  too  broad  and  far-reaching  in  its  scope  to  be 
included  within  the  narrow  confines  of  this  small  book, 
and  will  merely  be  summarized  with  maximum  brevity 
in  order  to  indicate  its  bearing  upon  our  title  subject. 

The  problem  is  the  prevention  of  the  silting-up  and  dis- 
location of  dredged  channels  and  areas  by  the  agencies 
of  sedimentation  and  scour.  Since  the  surest  cure  is  the 
removal  of  the  cause,  let  us  go  back  to  the  source  and 
review  hurriedly  the  marshalling  of  the  forces  of  nature 
for  the  destruction  of  the  works  of  man.  The  "rainfall," 
i.e.,  both  rain  and  melted  snow,  suffers  one  of ^three  fates: 
Part  of  it  is  evaporated,  part  penetrates  the  earth  surface 
to  become  "ground  water,"  which,  pursuing  subterranean 
courses,  eventually  reaches  the  river,  and  a  third  part 
remains  upon  the  surface,  constituting  the  surface  drainage 
and  creating  the  mountain  torrent  and,  finally,  through 
brook  and  creek,  reaches  the  river.  The  "run-off"  for  a 
particular  water-shed  is  the  rainfall,  less  the  evaporation, 
and  thus  comprises  both  surface  drainage  and  ground 
water.  The  percent  of  run-off  will  be  influenced  by  many 
factors,  principal  among  which  are  the  character  of  the 
soil,  the  slope,  the  vegetation,  the  climatic  conditions  and 
the  degree  of  concentration  of  the  precipitation  throughout 
the  year.  The  quantity  of  sediment  and  its  rate  of  prog- 
ress to  the  river  depends  upon  the  run-off  and  the  factors 
upon  which  the  run-off  is  dependent.  The  considerations 
thenceforth  are  the  ability  of  the  river  to  transport  the 
sediment  and  to  scour  its  banks  and  bed,  and  the  study  of 

125 


126  DREDGING  ENGINEERING 

the   laws   or    characteristics    governing   such   scour   and 
deposit. 

The  sediment-bearing  power  of  flowing  water  is  a  func- 
tion of  the  velocity  of  flow.  The  velocity  is  a  function 
of  the  slope  of  the  river  and  its  cross  section.  The  velocity 
is  not  uniform,  however,  throughout  the  cross  section.  Near 
the  banks  and  the  bottom  it  is  slower  because  of  frictional 
resistance,  and  the  maximum  velocity  will  obtain  somewhat 
above  the  mid  point  of  the  greatest  depth.  Sediment  may 
be  transported  either  in  suspension  or  by  being  rolled  along 
the  bottom.  The  heavier  the  material,  the  greater  is  the 
velocity  required.  A  velocity  that  is  just  sufficient  to 
carry  a  particular  class  of  material  is  unable  to  pick  up 
that  same  material  from  a  position  of  rest  on  the  bottom. 
The  deposition  of  sediment  is  caused  by  the  checking 
of  the  velocity  of  the  soil-bearing  current  due  to  obstruction 
or  change  in  grade  or  cross  section. 

/- 


FIG.  39. — Secondary  currents  in  the  cross  section  of  a  stream. 

The  principal  water  currents  existing  in  a  river  are  of 
two  kinds ;  first,  that  in  the  direction  of  the  river  slope  due 
to  gravity;  and,  second,  the  transverse  currents  caused  by 
change  of  direction  of  the  river's  course.  The  velocity  of 
the  flow  along  the  outer  bank'of  a  bend  is  naturally  greater 
than  on  the  inner,  and,  further,  the  axial  current  in  resist- 
ing the  change  in  direction,  creates  a  radial,  dynamic 
pressure  acting  toward  the  outer  bank  and  setting  up  cur- 
rents in  that  direction.  By  actual  observation  it  has  been 
found  that  these  cross-currents  move  toward  the  outer 
bank  near  the  river  surface  and  toward  the  inner  bank 
near  the  bottom  or  wetted  perimeter  of  the  cross  section, 
somewhat  as  shown  in  the  figure  39. 

Thus  it  is  the  outer  bank  of  a  river  on  a  bend  that  is  sub- 
jected to  scour  and  it  is  near  the  outer  bank  that  the 


PRELIMINARY  CONSTRUCTION  127 

deepest  water  will  be  found;  and  the  river,  by  simultane- 
ously building  up  by  deposit  the  inner  bank  of  the  bends, 
tends  ever  to  an  increasingly  winding  course.  The  fact 
that  rivers  of  the  least  slope  have  the  most  bends  is  due 
in  part  to  the  greater  relative  influence  of  these  cross- 
currents born  of  the  radial  pressure. 

The  weights  of  bodies  that  can  be  moved  by  the  pressure 
of  a  current  vary  as  the  sixth  power  of  the  velocity,  and  the 
diameters  of  the  bodies  as  the  square  of  the  velocity. 
Hence,  an  increase  in  velocity  causes  far  greater  increase  in 
transporting  capacity.  Mansfield  Merriman,  in  his  Treat- 
ise on  Hydraulics,  gives  the  following  table  of  approximate 
values : 

Velocity  of  0.25  feet  per  second  moves  fine  clay. 

Velocity  of  0.5  feet  per  second  moves  loam  and  earth. 

Velocity  of  1.0  feet  per  second  moves  sand. 

Velocity  of  2.0  feet  per  second  moves  gravel. 

Velocity  of  3.0  feet  per  second  moves  pebbles  1"  in  size. 

Velocity  of  4.0  feet  per  second  moves  spalls  and  stones. 

Velocity  of  6.0  feet  per  second  moves  large  stones. 

Rivers  are  opened  to  navigation  either  by  the  dredging 
of  a  deep  water  channel  in  their  beds  or  by  canalization. 
Minimum  first  cost  and  maintenance  expense  involve  a 
coordination  of  the  factors,  minimum  initial  excavation 
and  redredging.  Often,  the  natural  agencies  of  scour  and 
sedimentation  can  be  so  regulated  by  artificial  means  as  to 
reduce  the  cost  of  the  maintenance  of  the  dredged  cuts  by 
such  an  amount  as  to  warrant  the  cost  of  those  means. 
Such  regulation  takes  the  form  of  velocity  control  both  in 
amount  and  direction  by  artificial  changes  in  cross  section 
and  water .  course,  involving  the  construction  of  training 
and  spur  dikes  and  bank  protecting  structures.  To 
stabilize  the  course  of  the  river  and  therefore  the  channel 
location,  it  may  be  necessary  to  prevent  further  undulation 
of  course  by  protecting  the  outside  bank  of  a  bend  against 
further  recession.  This  may  be  accomplished  by  some  sort 
of  pavement  upon  the  exposed  slope  such  as  vegetation, 
brush  fascines,  mattresses,  sand  bags,  concrete  bags,  rip- 
rap or  an  apron  of  timber  or  concrete;  or  by  a  sheet  pile 


128  DREDGING  ENGINEERING 

revetment;  or  by  a  timber  or  concrete  bulkhead.  Or  the 
bank  may  be  retained  by  a  series  of  dikes  or  jetties  pro- 
jecting out  from  the  shore  into  the  waterway.  Such  dikes 
are  called  spur  dikes,  and  may  be  either  solid  or  permeable, 
i.e.,  they  may  be  an  impervious  current  stop  or  they  may 
have  openings  to  permit  the  passage  of  the  water  at  a 
reduced  velocity.  The  current  instead  of  attacking  the 
bank,  impinges  upon  the  spur  dikes  and  expends  its  energy 
in  the  creation  of  secondary  currents  and  eddies  until  its 
velocity  is  so  reduced  in  the  area  between  dikes  as  to  cause 
it  to  unload  its  suspended  material.  Again,  it  may  be 
desired  so  to  reduce  the  effective  width  of  the  river  as  to 
cause  an  increase  of  velocity  sufficient  to  create  scour  where 
sedimentation  had  previously  occurred  and  caused  ex- 
pensive redredging.  Spur  dikes  act  in  this  manner  to  a 
certain  extent,  but  more  often  a  training  dike,  which  is 
nearly  parallel  to  the  water  course,  is  employed.  The 
training"  dike  is  generally  of  much  greater  length  than  the 
spur  dike,  and  the  resultant  narrowing  of  the  stream  causes 
higher  than  original  velocities  for  the  same  discharge. 
Sometimes  an  island  is  so  situated  in  the  stream  that  it 
becomes  in  effect  a  training  dike  upon  the  connection  of  its 
upper  end  with  the  main  land. 

Man-made  dikes  are  of  several  principal  types  as  follows : 

1 .  Earth  Embankment,  with  or  without  protective  paving. 

2.  All-Stone  Embankment,  the  submerged  portion  usu- 
ally a  rip-rap  heap,  deposited  upon  a  brush  mattress,  and 
the  superstructure  either  rip-rap  of  larger  dimensions  or 
coursed  rubble. 

3.  Composite  Dike  of  rip-rap  substructure  surmounted 
by  a  gravity  wall  of  masonry  or  concrete  or  by  a  timber 
crib. 

4.  Single  row   of  sheet  piling,    driven   between   wales 
attached  to  plumb,  round  piles  and  braced  by  batter  piles. 
This  type  is  well  adapted  to  the  permeable  spur  duke,  as 
any  desired  percentage  of  permeability  may  be  obtained 
by  varying  the  width  of  aperture  between  successive  sheet 
piles. 


PRELIMINARY  CONSTRUCTION  129 

5.  Parallel  rows  of  sheet  piling,  as  above,  enclosing  a  fill 
of  suitable  material  and  tied  together  through  the  fill  with 
wire  rope,  chain  or  rods.     Additional  lateral  stability  may 
be  provided  by  spur  piles  in  both  directions  transversely 
in  the  fill. 

6.  Timber  Crib  with  good  heavy  filling. 

In  choosing  a  typical  section,  the  designer  should  have 
in  mind  the  forces  to  be  resisted,  i.e.,  the  " loading"  of  the 
dike.  Since  we  are  treating  only  of  such  dikes  as  are 
built  in  the  river  and  not  of  the  " levee"  which  confines  the 
river  itself,  the  hydrostatic  head  due  to  differences  of 
elevation  upon  the  two  sides  will  be  negligible,  and  the 
only  loads  to  be  considered  are  the  weight  of  the  structure 
itself,  current  scour,  wave  action  and  ice.  An  analysis 
of  wave  action  reveals  five  components: 

1.  The  normal  impact  due  to  the  waves'  momentum. 

2.  The  upward  force  parallel  to  the  face  of  the  dike 
tending  to  sheer  off  protruding  members. 

3.  Hydrostatic  head  due  to  the  increased  elevation  of  the 
water  surface. 

4.  Internal  pressures  due  to  the  imparting  of  impulses 
to  the  water  and  air  contained  within  the  interstices  of  the 
dike. 

5.  The  back  suctions  as  the  wave  recedes. 

Ice  may  create  a  very  considerable  thrust  against  the 
dike,  limitied  in  intensity  only  by  the  ultimate  crushing 
strength  of  the  ice. 

For  training  purposes,  in  localities  where  the  quarries  are 
available,  the  stone  dike  probably  gives  the  most  complete 
all-round  satisfaction.  A  brush  mattress  is  first  built 
somewhat  wider  than  the  asssumed  base  of  the  stone  pile 
and  is  sunk  between  guide  piles  by  loading  it  with  stone. 
For  the  substructure  and  the  core  of  the  superstructure, 
the  stone  my  range  in  size  from  one  man  stone  to  about 
four  ton  derrick  stone,  or  practically  the  run  of  the  quarry, 
but,  for  the  superstructure  facing  and  coping,  only  large 
sizes  should  be  used,  whether  the  section  be  rough  rip-rap 
or  squared  stone  rubble,  because  the  separate  units  must 


130  DREDGING  ENGINEERING 

have  sufficient  weight  to  resist  dislodgment  and  transporta- 
tion by  ice  and  waves.  For  the  same  reason,  the  super- 
structure is  laid  with  frequent  headers  to  give  the  necessary 
bond.  The  small  stone  is  deposited  by  stone  pans  or 
scale  boxes,  which  are  simply  rectangular  trays  open  at 
one  end  and  handled  by  a  derrick.  The  derrick  stone  is 
placed  by  the  use  of  chains  and  stone  hooks.  The  maxi- 
mum slope  of  the  underwater  portion  or  substructure  will 
be  1  to  1,  but  the  superstructure  wall  may  be  laid  up  more 
steeply,  if  desired,  to  economize  through  the  saving  in 
width  of  base  for  a  given  height.  For  estimating  purposes, 
the  body  of  the  dike  so  built  will  run  about  1J^  tons  of 
stone  per  cubic  yard  of  dike  for  granite,  gniess  or  rock  of 
a  similar  weight,  and  the  facing  and  topping  stone  about 
2  tons  to  the  cubic  yard. 

DIKES  FOR  IMPOUNDING  BASINS 

The  principal  component  structures  of  the  complete 
impounding  basin  are : 

1.  The  enclosing  structure  or  "dike." 

2.  The  " sluice"  or  " waste-weir"  for  the  discharge  of 
the  pumped  water. 

3.  The  occasionally  necessary  ditch,  canal  or  flume  for 
the  conveyance  of  the  effluent  from  the  sluice  to  its  source. 

The  type  of  dike  best  adapted  to  a  particular  case  and 
the  most  economical  design  and  method  of  construction 
will  depend  primarily  upon  the  location  of  the  proposed 
basin  with  respect  to  the  normal  beach  line,  i.e.  whether 
it  be  upon  the  land  or  in  the  water,  or  in  other  words, 
"dry"  or  submerged."  Obviously,  the  "dry  land"  dike 
is  the  simpler  and  less  expensive  general  type,  involving 
less  depth  of  fill,  and  less  potent  destructive  forces  to  resist. 
This  type  we  shall  first  consider. 

Dry  Dikes. — Most  commonly  the  Dry  Dike  is  an  earth 
embankment,  thrown  up  by  hand,  by  steam  shovels  or  by 
drag-scrapers.  The  nature  of  the  soil  will  influence  in  a 
measure  the  dimensions  of  the  bank,  but,  generally,  for 
a  height  of  more  than  6  feet,  the  top  width  should  be  not 


PRELIMINARY  CONSTRUCTION  131 

less  than  5  feet,  and  the  side  slopes  not  more  steep  than 
1  on  1%.  Aside  from  the  direct  hydrostatic  and  earth 
pressure  exerted  by  the  pumpings,  the  most  common  de- 
tructive  agents  threatening  the  stability  of  such  a  dike 
are  as  follows  :— 

1.  Scour. — A  concentration  of  flow  from  the  discharge 
pipe  along  a  bank  may  so  scour  and  disintegrate  the  in- 
side slope  as  to  endanger  the  structure.     Once  having 
broken  through,  the  escaping  water  widens  the  gap  and 
"  melts "   the  embankment   quite  rapidly.     The  remedy, 
of  course,  is  either  the  diversion  of  the  flow  by  shifting  the 
pipe  line  or  by  the  use  of  baffle  boards,  or  the  protection 
of  the  bank  with  brush  mattresses,  sand  bags  or  sheeting. 

2.  Wave  Action. — If  the  area  of  the  basin  be  such  as  to 
expose  a  considerable  expanse  of  water  to  the  winds,  the 
banks  may  be  reduced  by  wave  forces  unless  protected  as 
above. 

3.  Frost. — The   soil   comprising   an    embankment   built 
during  the  winter  may  be  so  frozen  as  to  cause  subsequent 
settlement  and  even  the  destruction  of  the  bank  after  the 
spring   thaw.     The   internal   portion  may  remain  frozen 
for  some  time  after  the  thaw  and  cause  trouble  when  not 
expected.     Again,  spring  thawing  of  the  frost  in  an  old 
bank  may  cause  such  softening  as  to  decrease  the  stability 
of  the  structure. 

4.  The  Muskrat. — Should  an  embankment  be  in  service 
for  longer  than  one  dredging  season,  it  may  become  neces- 
sary to  guard  against  the  ravages  of  the  muskrat,  which 
small  rodent  is  the  sworn  enemy  of  the  mud  bank.     As  soon 
as  the  mud  is  of  sufficient  consistency  to  "  stand  up  "  over 
his  operations,  he  excavates  tubular  tunnels  in  all  direc- 
tions until  the  dike  is  so  honeycombed  as  to  be  unsafe. 

Any  green  embankment  receiving  pumpings  should  be 
vigilantly  patrolled  until  such  time  as  it  is  known  to  be 
secure.  Vegetation  should  be  encouraged. 

Land  dikes  may  also  be  built  of  timber  sheeting :  either 
a  single  fence  preferably  of  two  layers  of  plank  with  lapping 
joints  and  banked  with  earth  on  one  side  to  lend  stability 


132  DREDGING  ENGINEERING 

and  prevent  leakage  or  two  parallel  lines  of  sheeting  about 
as  far  apart  as  their  height,  tied  together  with  wire  or  rods, 
and  filled  with  earth.  It  is  sometimes  possible  to  econo- 
mize on  the  banking  expense  by  raising  a  fill  a  few  feet  at 
a  time,  either  by  hand  made  embankments  of  small  size  or 
by  plank  retainers,  resulting  in  a  stepped  dike. 

Wet  Dikes. — Enclosing  the  "wet"  or  "submerged" 
impounding  basin  presents  rather  more  of  a  problem.  Here 
we  may  have  extensive  wave  action  with  which  to  contend, 
together  with  the  tides,  and  tidal  and  other  currents,  often 
increased  in  intensity  by  the  obstructive  nature  of  the  basin 
and  causing  rapid  scow  of  the  original  bottom  at  the  toe 
of  the  dike.  To  such  an  extent  has  this  happened  as  to 
establish  a  considerable  flow  under  the  dike  structure  be- 
tween the  contained  water  and  that  outside.  Moreover, 
as  previously  suggested,  the  dike  is  of  greater  total  height, 
due  to  the  addition  of  the  submerged  portion  and  must 
resist  pressures  of  greater  magnitude.  Dikes  for  this  pur- 
pose may  be  classified  under  one  of  the  six  following  captions : 

1.  The  Mud  Bank, 

2.  The  Mud  Fence  or  Single  Row  of  Sheeting, 

3.  Parallel  Rows  of  Sheeting,  filled  with  Earth. 

4.  The  Timber  Crib, 

5.  The  Stone  Dike, 

6.  The  Bulkhead  or  Marginal  Wharf . 

The  first  is  simply  a  mound  of  earth  cast  up  by  a  long- 
boom  grapple  dredge  from  the  bottom  soil.  Although 
often  employed  with  varying  degrees  of  success,  it  is 
frequently  of  doubtful  stability  and  permanence.  The  ma- 
terial of  which  it  is  built  is  saturated  and  suffers  there- 
from in  angle  of  repose.  It  may  be  easily  so  soft  as  to  be 
incapable  of  "standing  up"  above  the  water  surface. 
Unless  protected  upon  its  slopes  by  more  resistant  material, 
it  is  easily  scoured  and  melted  away  by  wave  and  tide 
action.  The  depth  of  water  in  which  it  is  to  be  built  is  an 
important  factor. 

The  mud-fence  is  simply  a  tight  vertical  wall  of  continuous 
sheet  piling,  driven  between  guide  wales,  supported  at  in- 


PRELIMINARY  CONSTRUCTION 


133 


tervals  by  round  piles  and  stabilized  against  the  thrust 
of  the  pumpings  by  spur  or  batter  piles  or  by  tie  rods  and 
anchorages.  The  sheeting  may  be  a  single  thickness,  or 
two  layers,  shiplapped,  or  Wakefield  piling,  depending 
upon  the  degree  of  tightness  necessary  to  prevent  leakage, 
as  determined  by  the  nature  of  the  backing  and  pumpings. 
Wakefield  is  a  built  up  tongue  and  groove  piling,  consisting 
of  3  planks  fastened  together  as  in  Fig.  40.  The  fastening 
is  effected  by  machine  bolts,  by  wire  spikes  driven  through 


-7- 


D 


Shiplapped  Wakefield  Tongue  *  Groove 

Bplined  Dovetailed 

FIG.  40. — The  mud-fence. 

and  clinched  or  by  a  combination  of  the  two.  A  fourth 
method  employs  so-called  splines,  set  in  grooves  in  the 
sheet  pile  proper  or  nailed  to  the  edge  thereof.  The  dimen- 
sions of  section  and  the  length  of  the  sheeting  will  depend 
upon  the  nature  of  the  bottom,  the  depth  of  water,  the 
height  of  the  structure  and  the  filling  conditions.  The  size 
of  the  wales,  spacing  of  round  piles  and  anchorages  or  spur 
piles  are  determined  from  a  study  of  the  pressures  acting 
upon  the  fence,  by  methods  which  will  be  outlined  subse- 
quently. The  tie  rods  should  be  upset  and  provided  with 
turnbuckles,  by  which  an  initial  stress  is  set  up  in  them 


134  DREDGING  ENGINEERING 

to  overcome  the  inaccuracies  of  framing  the  spur  pile 
connection  and  to  bring  the  surfaces  into  full  bearing  so 
that  the  resistance  of  the  anchorage  may  be  developed  in 
the  beginning  before  distortion  takes  place.  This  is  very 
necessary  to  the  preservation  of  the  alignment  of  the 
retainer.  The  tie-rod  washers  are  theoretically  propor- 
tioned to  distribute  the  bearing  sufficiently  to  obviate 
local  failure.  Before  commencing  to  pump  into  the  basin, 
it  is  often  advisable  to  back  the  mud  fence  with  a  bank 
of  mud,  or,  if  practicable,  of  more  suitable  material. 


FIG.  41. — Long  boom  banking  machine  (Delaware  Dredging  Co.)  filling  a  timber 

crib  dike. 


Conditions  of  greater  depth  and  softer  bottom  may  re- 
quire the  construction  of  a  heavier  and  more  expensive 
structure.  Parallel  mud  fences,  facing  in  opposite  direc- 
tions, almost  as  far  apart  as  their  height  above  the  mud, 
tied  across  and  braced  by  spur  piles  between  them  and  filled 
with  earth,  form  a  substantial  retainer  which  may  prove 
best  adapted  to  a  particular  problem. 

The  timber  crib  affords  a  useful  expedient  where  the 
bottom  is  either  of  rock,  prohibiting  the  penetration  of 
piles,  or  of  soft  mud,  affording  scant  lateral  resistance  for 
the  sheet  pile  structure.  A  description  of  it  is  hardly 


PRELIMINARY  CONSTRUCTION  135 

necessary.  It  is  simply  a  basket  of  squared,  flitched  or 
round  timbers,  successive  layers  of  which  are  laid  hori- 
zontally at  right  angles  to  each  other,  notched  and  spiked 
at  the  intersections,  forming  a  cellular  box  which  is  sub- 
sequently filled  with  earth  or  preferably  heavier  material. 
It  may  be  floored  and  lined  with  plank  to  secure  greater 
fill-tightness,  or  the  floor  may  be  omitted  and  the  lining 
planks  driven  down  as  sheeting  into  the  mud  below  the 
crib.  The  bottom  having  been  levelled  as  much  as  possible 
by  dredging,  the  crib  is  built  to  fit  the  prepared  surface. 
It  may  be  constructed  in  the  dry  in  sections  and  sunk  by 
filling,  or  built  up  floating  in  the  water,  sinking  under  its 
own  weight  as  the  walls  rise,  and  guided  to  the  bed  by  guide 
piles.  The  width  of  the  crib  at  the  base  is  generally 
almost  as  great  as  the  height.  The  rear  face  may  be 
stepped,  and  the  front  face  battered. 

The  stone  dike,  except  under  the  most  favorable 
conditions  of  market  and  working  facilities  will  rarely 
prove  economical.  Little  more  need  be  said  about  it  in 
this  connection.  It  is  generally  quite  pervious  to  the 
pumpings,  requiring,  for  tightness  a  mud  or  other  suitable 
backing. 

By  the  bulkhead  or  marginal  wharf  is  meant  a  retaining 
structure  permitting  deep  water  at  its  face  for  vessel 
flotation.  It  is  not  essentially  a  dike,  but  is  mentioned  here 
for  the  reason  that,  under  certain  conditions,  the  enclosing 
of  an  impounding  basin  by  such  a  structure,  the  province 
of  which  is  not  only  to  serve  as  a  dike,  but  also  to  make  the 
property  available  for  wharfage  purposes,  may  prove  to  be 
the  economical  solution.  The  foregoing  dikes,  such  as  the 
timber  crib  or  the  parallel  rows  of  sheeting  will  often  in- 
volve considerable  cost,  entailing  the  expenditure  of  a  large 
sum  of  money  for  a  structure,  the  sole  function  of  which  is 
to  serve  as  a  dike,  so  that  the  enhanced  value  of  the  bulk- 
headed  property  may  warrant  the  greater  initial  outlay. 
If  the  basin  is  ultimately  to  be  used  as  a  wharf  property, 
it  is  obviously  in  the  interest  of  economy  to  accomplish 
this  end  in  one  operation  rather  than  subsequently  to  sup- 


136  DREDGING  ENGINEERING 

plement  a  dike  with  a  wharf  structure,  unless  there  be 
attenuating  circumstances. 

Spur  Piles. — Of  the  above  six  types  of  wet  dikes,  it  will 
readily  be  seen  that  Numbers  1,  4  and  5,  and  Number  3 
partially,  are  gravity  retainers,  and  that  Numbers  2  and 
6,  and  to  some  extent  Number  3,  employ  the  spur  or  batter 
pile  to  resist  the  overturning  tendency.  It  would  appear, 
therefore,  that  the  subject  of  spur  piles  is  of  sufficient 
importance  to  merit  discussion  here.  Moreover,  the  di- 
version, if  it  be  such,  is  further  excused  by  the  tendency 
among  some  contractors  and  even  engineers  to  give  inade- 
quate attention  to  the  spur  pile  details  both  in  design  and 
construction.  Not  only  is  this  true  of  the  dike,  but  of 
bulkheading  structures  generally,  wherever  the  spur  pile 
is  employed,  and  many  failures  may  be  attributed  to  this 
fact  alone.  In  the  first  place,  because  of  the  expense  of 
providing  special  inclined  ways  for  driving  the  batter 
piles,  they  are  frequently  driven  by  tilting  up  the  leads  of 
vertical  pile  drivers,  resulting  in  insufficient  batter  to 
develop  the  horizontal  resistance  required.  An  inclina- 
tion of  30  degrees  to  the  vertical  is  entirely  practicable  in 
the  great  majority  of  cases  and  should  be  insisted  upon. 
Again,  whether  through  failure  to  realize  fully  the  nature 
and  extent  of  the  forces  acting,  or  through  the  desire  to 
economize  in  framing  and  hardware  costs,  the  head  of  the 
spur  pile  is  often  inadequately  retained  to  develop  the  full 
horizontal  resistance. 

Let  us  discuss  for  the  moment  the  spur  pile  in  general. 
Assume  that  a  pile  is  driven  on  a  batter  of  1  to  2  to  such 
penetration  as  to  develop  a  safe  axial  bearing  value  of  12 
tons  or  24,000  pounds.  The  vertical  component, .  Fig.  42, 
is  about  21,400  pounds  and  the  horizontal  10,700  pounds. 
The  pile  then  is  capable  safely  of  resisting  a  horizontal 
thrust  of  10,700  pounds  if — and  herein  lies  the  difficulty— 
the  head  of  the  pile  is  held  down  by  a  force  equal  to  the 
vertical  component,  21,400  pounds.  In  bulkheads  of  the 
relieving  platform  type,  this  downward  resistance  to  the 
upthrust  of  the  spur  pile,  is  furnished  wholly  or  in  part  by 


PRELIMINARY  CONSTRUCTION 


137 


the  weight  of  the  fill  upon  the  platform,  but  in  other  types 
of  retaining  structure,  such  as  the  cases  in  point,  or  in 
marginal  wharves  of  the  above- water  pile  and  platform 
design,  it  becomes  necessary  to  depend  upon  the  resistance 
to  pulling  of  a  vertical  pile  to  which  the  spur  pile  is  attached, 
and  it  is  this  attachment  which  merits  emphasis. 

If  framed  as  in  Fig.  40,  page  133,  the  depth  of  "gain" 
in  the  plumb  pile  must  be  sufficient  to  give  a  horizontal 
bearing  surface  of  such  area  as  to  keep  the  unit  bearing 


10,700 


21,400 


FIG.  42. — Spur  pile  resistances. 

within  the  allowable  limit.  If  the  safe  end-grain  bearing 
value  is  1000  pounds  per  sq.  in.,  the  area  required  for  the 
above  case  is  21.4  sq.  in.,  which,  in  a  14  inch  pile,  will 
obtain  at  a  depth  of  about  3  inches.  Moreover,  the  length 
of  plumb  pile  above  the  notch  must  be  great  enough  to 
preclude  failure  by  shearing  with  the  grain.  It  is  not  always 
practicable,  however,  to  drive  spur  and  plumb  pile  in  such 
relative  position  as  to  permit  a  fastening  of  this  kind.  In 
many  instances,  the  batter  pile  must  be  placed  alongside  a 
bent  of  vertical  piles,  in  which  position,  the  problem  of 


138 


DREDGING  ENGINEERING 


sustaining  its  upward  reaction  often  presents  difficulties. 
It  is  manifestly  not  enough  simply  to  bolt  the  spur  and  plumb 
pile  together  horizontally  at  their  intersection,  because, 
even  though  the  bolt  be  of  sufficient  size  to  take  the  shear, 
the  fastening  will  fail  through  the  bending  of  the  bolt  and 
the  crushing  of  the  wood  fibres  in  the  bolt  hole  at  the  point 
of  contact  of  the  two  piles.  The  most  convenient  and  effi- 
cacious arrangement  of  this  detail  in  high  timber  platform 


Fly.  a 


Fifj.b 


FIG.  43. — Spur  pile  detail. 

bulkheads,  in  so  far  as  the  vertical  forces  are  concerned, 
is  the  capping  of  the  spur  piles  with  a  timber  running  across 
the  bent  cap  and  called  the  "spur  cap, "  which  transmits 
the  upthrust  to  one  of  the  plumb  bearing  piles  of  the  bent, 
through  a  pair  of  strap  bolts,  Fig.  43.  Even  here,  it  fre- 
quently happens  that  the  strap  bolts  themselves  and  their 
fastening  to  the  bearing  pile  is  not  given  sufficient  attention. 
In  the  first  place,  if  the  reaction  is  to  be  transmitted  to  the 
plumb  pile  through  the  tension  in  two  bolts,  one  on  each 
side  of  the  pile  (as  is  generally  required),  the  spur  cap  must 


PRELIMINARY  CONSTRUCTION  139 

be  continuous  over  spur  pile  and  bent  cap,  breaking  joint 
somewhere  between  bents.  In  the  second  place,  the  bolts 
must  be  everywhere  of  sufficient  net  section  to  take  the 
tension,  and,  finally,  the  attachment  to  the  bearing  pile 
must  be  adequate  in  shear  and  bearing  both  in  the  metal 
and  timber.  It  is  the  latter,  the  bearing  of  the  bolts  in 
the  holes  through  the  pile,  that  proves  most  troublesome. 

Referring  to  the  assumed  problem,  the  stress  in  each 
strap  bolt  is  10,700  pounds,  requiring  10.7  sq.  in.  of  end 
grain  bearing  for  a  unit  value  of  1,000  pounds  per  sq.  in. 
Using  %  in.  bolts  through  the  pile  from  strap  to  strap, 
this  means  that  three  would  be  required,  since  it  is  hardly 
safe  to  assume  that  a  length  of  hole  more  than  four  times 
the  diameter  of  the  bolt  is  effective  in  taking  the  bearing 
because  of  the  bending  of  the  bolt.  It  is  next  to  impossible, 
however,  to  bore  three  holes  through  a  14  in.  pile  such  that 
they  will  be  opposite  the  bolt  holes  in  each  strap  on  each 
side.  Lag  screws  may  be  substituted,  but  are  not  wholly 
satisfactory.  A  scheme  devised  by  the  author  employs 
a  single  heavy  bolt,  about  \y±  in.  in  diameter,  and  two  cast 
iron  spools  to  furnish  the  required  bearing  surface,  one 
at  each  side,  Fig.  43,  page  138.  The  spools  are  cast  with 
a  slight  taper  and  driven  snugly  kinto  tholes  bored  by  a 
3}i  in.  auger  to  the  depth^required.  The  pile  surface 
under  the  straps  must  be  freed  of  bark  before  placing 
them.  This  makes  a  positive  and  inexpensive  fastening, 
easily  installed. 

Pressures  on  Wet  Dikes. — In  the  design  of  dikes  for  en- 
closing a  submerged  basin,  there  are  three  conditions  of 
loading  to  be  considered,  although  all  three  may  not  neces- 
sarily obtain  in  all  dikes,  not  in  all  portions  of  the  length 
of  one  specific  dike. 

They  are  as  follows: 

Case  I. — The  pressure  exerted  by  the  dredged  material, 
when  in  a  saturated  or  semi-fluid  condition,  while  the 
hydraulic  filling  is  still  in  progress. 

Case  II. — The  hydrostatic  pressure  of  a  head  of  con- 
tained water  alone,  which,  having  dropped  its  suspended 


140  DREDGING  ENGINEERING 

matter,  is  still  retained  within  the  basin  up  to  the  elevation 
of  the  crest  of  the  waste- weir. 

Case  III. — The  pressure  of  the  ultimate  loading,  when 
the  completely  filled  basin,  having  drained  and  dried  out, 
is  subject  to  the  weight  of  surcharge  or  of  extraneous  dead 
or  live  loads. 

In  all  three  cases,  the  internal  pressure  is  partly  counter- 
acted by  the  external  hydrostatic  pressure,  the  critical 
loading  obtaining  when  the  latter  is  a  minimum  at  the  time 
of  extreme  low  water. 

In  the  first  case,  the  controlling  factors  are  the  nature 
of  the  material  dredged,  the  size  and  shape  of  the  basin 
and  the  manner  and  rate  of  filling.  Usually,  in  the  process 
of  filling  the  basin,  the  discharge  pipe-line  is  lengthened 
by  the  addition  of  pipe  sections  only  as  the  fill  at  its  mouth 
is  brought  up  to  final  grade,  resulting,  after  a  time,  in  a 
basin  filled  to  grade  and  dry  in  the  vicinity  of  the  point  of 
initial  discharge,  from  which  the  surface  of  the  fill  slopes 
gently  down  beneath  the  contained  water  to  the  original 
bottom  in  the  remoter  portions  of  the  basin,  where  simply 
a  head  of  water  exists  from  the  original  bottom  of  the  basin 
to  the  elevation  of  the  height  of  the  crest  at  the  sluice. 
As  the  pumping  continues,  the  inclined  surface  of  the  fill 
progresses  toward  the  sluice,  tending  constantly  to  decrease 
the  surface  area  of  the  contained  water  and  thus  necessi- 
tating raising  the  elevation  of  the  water  by  the  placing  of 
additional  weir  boards  in  the  sluice-box  in  order  that  the 
submerged  or  impounding  area  may  remain  sufficiently 
large  to  fulfill  its  function  of  precipitation  necessary  to 
a  reasonably  clear  effluent.  If  the  dredgings  consist  en- 
tirely of  alluvial  silt  or  river  mud,  this  surface  slope  will 
be  very  flat  and  a  large  impounding  area  required.  If  the 
material  be  of  uniformly  heavy  material,  the  slope  will  be 
sharper,  permitting  a  smaller  basin.  Finally,  if  the  p amp- 
ings  be  a  mixture  of  mud  or  silt  and  some  heavier  material 
such  as  coarse  sand,  the  effect  of  the  hydraulic  handling 
will  be  to  segregate  the  two,  the  sand  being  deposited 
quickly  near  the  mouth  of  the  discharge  pipe,  so  that  only 


PRELIMINARY  CONSTRUCTION  141 

the  more  remote  portions  of  the  dike  will  receive  the  greater 
thrust  of  mud  alone.  Even  if  mud  be  the  sole  element  of 
the  pumpings,  the  area  of  the  basin  may  be  so  great  and 
the  rate  of  filling  so  slow  that,  before  the  fill  attains  its 
final  elevation,  the  earlier  pumpings  near  the  bottom  will 
have  had  opportunity  to  compact  or  set  to  a  certain  extent, 
acquiring  an  appreciable  angle  of  repose  and  lessening  the 
pressure.  More  especially  will  this  be  true  if  the  basin  be 
drained  at  frequent  intervals. 

The  greatest  possible  pressure  of  the  first  condition  of 
loading  is  that  resulting  from  a  filling  of  what  is  virtually 
liquid  mud.  River  mud  submerged  or  completely  satu- 
rated has  an  angle  of  repose  of  zero,  so  that  it  exerts  a 
fluid  pressure  which  is  much  greater  than  that  of  water 
because  of  the  greater  weight  of  the  liquid.  Whether  or 
not  the  dike  will  be  subjected  to  a  total  head  of  liquid  mud, 
the  engineer  must  predetermine  from  a  study  of  the 
foregoing  elements. 

The  pressure  of  Case  II  can  be  the  critical  loading  only 
when  the  pumped  material  is  of  such  heavy  nature  as  to 
deposit  rapidly  and  exert  a  horizontal  thrust  less  than  that 
of  the  head  of  contained  water.  In  such  case,  the  maxi- 
mum head  will  obtain  when  the  basin  is  in  an  advanced 
stage  of  completion  and  the  weir  has  been  raised  to  or 
nearly  to  its  full  height. 

The  loading  of  Case  III  presupposes  the  subsequent 
use  of  the  fill  for  commercial  purposes,  entailing  the  placing 
of  heavy,  quiet  or  moving  loads  upon  its  surface  close  to 
the  dike. 

The  pressures  resulting  from  the  three  conditions  of 
loading  will  now  be  investigated. 

Case  I. — Let  us  assume  a  dike  built  in  8  feet  of  water  and 
rising  10  feet  above  the  surface,  subjected  to  a  loading  of 
liquid  mud.  We  then  have  a  retainer  separating  two  fluids 
of  different  weights  and  heads,  the  mud  emulsion  on  one 
side  weighing  100  Ibs.  per  cu.  ft.  with  an  18  ft.  head,  and 
water  on  the  outside  weighing  60  Ibs.  per  cu.  ft.  with  a  head 
of  8  ft.,  Fig.  44. 


142 


DREDGING  ENGINEERING 


in  which 

Pm  =  the  resultant  internal  mud  pressure  per  ft.  of 

dike. 
Pw  =  the  resultant  external  water  pressure  per  ft.  of 

dike. 
Then    Pm  =  %  X  100  X  I82  =  16,200  Ibs.,   acting  at  a 

18 
distance  -~  or  6  ft.,  above  the  bottom. 

Pw  =  M  X60  X  82  =  1,920  Ibs.,  acting  at  a  distance 

o 

-  =  2'  -8"  above  the  bottom. 


r 


(8  X60) 
FIG.  44. — Dike  loading — Case  I. 

Case  II. — Now  suppose  that  the  critical  loading  of  the 
same  dike  be  that  of  the  second  condition,  i.e.,  hydro- 
static pressure  on  both  sides.  The  resultant  internal 
pressure  per  foot  of  dike  becomes  H  X  60  X  182  =  9,720 
Ibs.,  the  external  pressure  and  the  point  of  application  of 
each  remaining  the  same  as  in  Case  I. 

Case  III. — Finally,  let  us  apply  the  third  or  ultimate  con- 
dition of  loading  to  the  dike,  assuming  that  the  filling  be  a 
mixture  of  gravel,  sand  and  clay  and  that  the  live  load 
surcharge  equals  600  Ibs.  per  sq.  ft.  The  table,  page  143, 


PRELIMINARY  CONSTRUCTION 


143 


taken  from  the  American  Civil  Engineers'  Hand  Book 
(Merriman),  gives  the  weight,  slope  and  angle  of  repose  of 
various  materials  both  as  loose  earth  in  air  and  as  excavated 
material  dumped  into  water.  Using  the  following  notation : 


Loose  Earth  in  Air 

Excavated  Materials  Dumped  into 
Water 

Kind  of  Material 

Slope  of 
Repose 

Angle  of 
Repose 

Weight  Ibs. 
per  cu.  ft. 

Slope  of 
Repose 

Angle  of 
Repose 

Weight  Ibs. 
per  cu.  ft. 

Clean  sand  

1.5  to  1 

33°—  41' 

90 

2  to  1 

26°—  34' 

60 

Sand  and  clay  

1.33  to  1 

36°—  53' 

100 

3  to  1 

18°—  26' 

65 

Clay  —  dry  

1.33  to  1 

36°—  53' 

100 

3M  to  1 

15°—  57' 

80 

Clay  —  (damp) 

(plastic)  

2  to  1 

26°—  34' 

100  J 

Clean  gravel  

1.33  to  1 

36°—  53' 

100 

2  to  1 

26°—  34' 

60 

Gravel  and  clay,  i  . 

1.33  to  1 

36°—  53' 

100 

3  to  1 

18°—  26' 

65 

Gravel,    sand    and 

clay  

1  33  to  1 

36°  —  53' 

100 

3  to  1 

18°  —  26' 

65 

Soil  

1  33  to  1 

36°  —  53' 

100 

3H  to  1 

15°  —  57' 

70 

Soft    rotten  rock.. 

1.33  to  1 

3G°—  53' 

110 

1  to  1 

45°—  0' 

65 

Rip  rap  

1  to  1 

45°—  0' 

65 

River  mud  

oc  to  1 

0—0 

90 

Wa  =  weight  per  cubic  foot  in  air 
Ww  =  weight  per  cubic  foot  in  water 
(j>a    =  angle  of  repose  in  air 
(j)w    =  angle  of  repose  ir  water 

The  quantities  for  a  mixture  of  gravel,  sand  and  clay  are: 

Wa  =  100  Ibs. 
Ww  =  65  Ibs. 
0,    =  36°53' 
0W    =  18°26' 

The  most  workable  formula  for  earth  pressure,  P,  per  foot 
of  dike  and  one  whose  degree  of  accuracy  is  consistent  with 
that  of  the  other  factors  of  our  problem  is 

P  =  \i  wh2  tan2  (45°  -  H0) 

which  assumes  that,  were  the  dike  to  be  removed,  the 
backing  would  fail  by  parting  along  the  plane  ac,  Fig.  45a, 
called  the  plane  of  rupture,  and  making  an  angle  with  the 
vertical  of  45°  — >£<£,  so  that  the  horizontal  thrust  against 


144 


DREDGING  ENGINEERING 


the  dike  is  caused  by  the  tendency  of  the  wedge  abc  to 
slide  upon  the  plane  ac,  and  is  the  horizontal  component 
of  a  force  paralleling  the  plane  of  rupture,  the  vertical 
component  of  which  is  the  weight  of  the  sliding  wedge  abc. 
For  one  horizontal  foot  of  wall,  the  quantities  involved 
are: 


=  ~  X  h  tan   (45°  — 


Area  of  sliding  wedge  abc 

mt 

=  Y^W  tan  (45° 

Weight,  Wy  of  sliding  wedge  abc  =  %whHan  (45° 
Horizontal  thrust,  P,  =  W  tan  (45°  - 

(45°  - 


FIG.  45. — The  sliding  wedge. 

P  may  be  found  graphically,  Fig.  45&,  by  laying  off  to 
scale  on  a  vertical  line  the  weight,  TF,  of  the  sliding  wedge 
and  the  angle  of  rupture,  45°  —  K0>  then  closing  the 
triangle  by  a  horizontal  line,  the  length  of  which  equals  the 

thrust,  P. 

°      ' 
In  the  problem  assumed,  45°  - 


=  26°34/  and  45°  - 


45°  — v- 


(18°26') 


45°  - 


=  35°47'.     The 


conditions  then  are  as  shown  in  Fig.  46. 


PRELIMINARY  CONSTRUCTION 


145 


The  weight  of  the  sliding  wedge  is  made  up  of  three  parts : 

(1)  the  area  abc  in  sq.  ft.  X  Ww 

(2)  the  area  bdefc  in  sq.  ft.  X  Wa 

(3)  the  length  df  in  ft.  X  600. 

and,  if  the  figure  is  drawn  accurately  with  scale  and  pro- 
tractor, the  areas  may  be  found  from  scaled  dimensions, 


M.L.W.. 


Fi«'a  Fig.b 

FIG.  46. — Dike  loading — CASE  III. 

The  total  weight  of  the  composite  sliding  wedge  and  the 
angle  of  rupture  of  the  lowest  stratum  determine  the  magni- 
tude of  the  horizontal  thrust.  The  total  weight  is 

Submerged  stratum  abc  =1510  Ibs. 

Dry  stratum  bdefc         =  8300  Ibs. 

Live  load  df  X  600  =  6480  Ibs. 


Total          16,290  Ibs. 
Then,  graphically,  as  before, 

P  =  11,700  Ibs. 

In  this  instance,  the  solution  yields  the  net  thrust,  as 
the  water  pressure  has  already  in  effect  been  deducted  by 

10 


146  DREDGING  ENGINEERING 

reason  of  the  fact  that  the  quantities  used  for  the  submerged 
stratum  were  submerged  quantities.  The  pressure  to  be 
resisted  by  the  wall  is,  therefore,  11,700  Ibs. 

The  formulae  will  of  course  yield  the  same  answer  with- 
out graphical  aid. 

Design.  —  In  those  dikes  not  of  the  gravity  type,  it  now 
remains  to  ascertain  the  stresses  in  the  sheet  piling  and  the 
proportion  of  the  thrust  transmitted  thereby  to  the  above 
water  structure.  The  most  convenient  treatment  is  by 
the  method  of  equivalent  fluid  pressure,  i.e.  the  determina- 
tion of  the  weight  of  a  hypothetical  fluid  which  will  pro- 
duce the  same  thrust  P  both  in  magnitude  and  point  of 
application.  Resultant  earth  pressures  act  at  a  point 
somewhere  between  one-third  and  one-half  the  height  of  the 
wall  from  the  bottom.  Where  the  live  load  surcharge  is 
considerable,  the  point  of  application  may  be  safely  as- 
sumed at  0.4  the  height.  In  our  problem,  this  point  will 
be  0.4  X  18  or  7.2  feet  from  the  bottom.  Call  it  7  feet  even. 
Hydrostatic  resultant  pressures,  however,  act  at  one-third 
of  the  height.  The  head,  therefore,  of  the  so-called  equiva- 
lent fluid  will,  in  this  case,  be  3  X  7  X  or  21  feet,  and  the 


w  w 

resultant  pressure  =  -^-  =  ---  ~  —   =  220.  5w.    Equating 

this  to  the  above  value  of  P  and  solving  for  w,  we  find 
that  a  fluid  weighing  53  pounds  per  cubic  foot  will  exert 
an  equivalent  thrust.  The  unit  pressure  at  the  base  of  the 
wall  =  wh  =  53  X  21  =  1110  Ibs.  Again,  in  the  Fig.  46, 
lay  this  off  to  scale  and  draw  the  triangle  of  fluid  pressures, 
agh,  from  which  the  unit  pressure  at  any  point  of  the  height 
may  be  scaled  and  the  stresses  in  the  structure  analyzed. 
In  the  case  of  the  mud  fence,  the  first  step  from  this  point 
will  be  the  determination  of  the  proportion  of  the  thrust  car- 
ried by  the  tie  rods  and  spur  piles,  whence,  knowing  this  re- 
action, the  bending  moment  in  the  sheet  piling  maybe  found. 
As  a  more  complete  illustration  of  the  problem  subse- 
quent to  the  evolution  of  the  pressure  triangle  of  the  equi- 
valent fluid,  let  us  discuss  the  case  of  the  relieving-platform, 
sheet-pile  bulkhead,  Fig.  47. 


PRELIMINARY  CONSTRUCTION 


147 


The  sheeting  is  a  vertical  beam  fixed  at  the  lower  end  at 
A  and  supported  at  the  top,  B.     The  point  of  fixture,  A, 


FIG.  47. — Platform  bulkhead. 

will  be  at  or  a  short  distance  below  the  mud  line,  depending 
upon  the  resistant  qualities  of  the  material  comprising  the 


t 


.070  Wl- 


ff 


Loading 


5$  W 
Shear 


Moment 


Elastic  Curve 


060  Wl 


FIG.  48. — Shear  and  moment  in  sheet  piling. 

bottom.     For  the  mixture  of  gravel,  sand  and  clay,  we 
will  assume  a  value  of  2  feet.     The  beam  AB  is  resisting  the 


148  DREDGING  ENGINEERING 

trapezoid  of  pressures  abde,  and  the  problem  is  to  find  the 
reactions  at  A  and  B  and  the  bending  moment  in  the  sheet- 
ing. To  this  end,  consider  the  beam  loading  in  two  parts; 
1st,  the  uniform  load,  bdfa,  and,  2nd,  the  uniformly  varying 
load  0  to  fc  pictured  by  the  triangle  def,  and  ascertain, 
independently,  the  end  reactions,  the  moments  and  shear 
in  the  beam  due  to  each  load.  Then  by  combining  the 
two,  the  critical  stresses  in  the  sheeting  may  be  found. 
Fig.  48a,  gives  the  diagrams  for  the  uniform  load,  abdf, 
and  Fig.  486,  for  the  variable  load,  def.1 

1  Mechanics  of  a  Beam,  of  length  I,  fixed  at  one  end  and  supported  at 
the  other,  carrying  a  total  load,  wl,  varying  uniformly  from  zero  at  the 
supported  end  to  2w  (twice  the  average  load  per  foot,  w)  at  the  fixed  end. 

If  Ri  =  the  reaction  at  the  left  or  supported  end,  the  bending  moment, 
Mx,  at  any  section,  distant  x  from  Ri,  is 

Mx  =  RlX  -  -~  (1) 

The  equation  of  the  elastic  curve,  using  the  conventional  notation,  is 

.       */g-"  /  (2) 

whence  El  *£  _  RiX  _.  »*  (3) 

Integrating  (3)—  El  ^  =  #1  ^  -  ^  +  C1  (1) 

Integrating  (^—Ely  =  Ri  %  -  ~  +  C,x  +  C2 

Since  x  =  0  when  y  =  0,  the  constant  C2  =  0. 

7  dy       A  ,  ~        wl3       Ril2 

and  when  x  =  I,  ~  =  0.  .*.  the  constant  C\  =  y^  --  o~ 


,    vj     _  wx* 

~G~    ~  60Z  4"~12~         ~2~ 

60  Ely  =  lO^i  (x3  -  Wx)  -  w(j  -  5l3x\  (5) 

In  (5)  when  x  —  Z,  y  =  0. 

wl 

whence  Ri  =  -=-  (6) 

o 

Substituting  (6)  in  (1)  M  =  ^  -  ^f  (7) 

o  ot 

dM  _  wl       wx2  _  n 
dx~  ~  6"      ~T 

.'.  Max.  positive  moment  occurs  when  x  =  —  -p.  = 

0.447  I  (8) 

Substituting  (8)  in  (7),  max.  pos.  M  =  +  0.0596  wl2 

2 
Max.  negative  moment  (x  =  Jin  (7))  M  =  —  ^  wl2 

Point  of  inflection  (M  =  0  in  (7))  is  distant  0.775Z  from  RI 


PRELIMINARY  CONSTRUCTION  149 

Bearing  in  mind,  then,  that  the  W  ofathe  first  condition, 
which  we  will  call  TFi,  equals  bd  =  af,  and  the  W  of  the 
second,  called  TF2,  equals  the  average  unit  load,  or 
the  combined  quantities  for  the  total  load  abdef  are: 

3  1 

Reaction  at  B       =  -WJ  +  ^  WJ, 

5  4 

Reaction  at  A       =3^  +  5  w*1 

Max.  neg.  B.  M  .  =  \  Wtf  +  ^  W2l2 

o  1O 

Max.  pos.  B.  M.  =         TFiZ2  +  0.06  W,P 


the  last  being  not  absolutely  true,  due  to  the  fact  that  the 
point  of  max.  pos.  B.  M.  is  not  coincident  in  the  two  cases. 
However,  it  is  sufficiently  accurate  to  be  consistent  with 
the  other  assumptions,  and  the  error  is  on  the  side  of  safety. 
The  total  pressure  resisted  by  the  bulkhead  structure  is 
the  sum  of  the  reaction  at  B  as  above,  plus  the  triangle 
bed.  Whereas  the  assumption  that  the  point  of  application 
of  the  resultant  pressure,  for  the  usual  average  live  load 
surcharge,  is  0.4  of  the  height  from  the  base  is  sufficiently 
accurate  for  the  design  of  the  structures  cited,  for  the 
larger  sea  walls,  an  investigation  of  the  location  of  that 
point  becomes  necessary.  Such  a  discussion,  however,  is 
neither  commensurate  with  the  scope  of  this  book,  nor 
apropos  of  our  subject  title.  Mr.  S.  W.  Hoag,  in  the 
Proceedings  of  the  Municipal  Engineers  of  the  City  of 
New  York,  1905,  describes  in  detail  the  two  methods  of 
pressure  calculation  used  by  the  Department  of  Docks  in 
New  York.  The  more  recent  of  the  two  reduces  the  several 
component  substances  of  the  sliding  wedge  to  a  single 
homogeneous  material,  that  at  the  base  of  the  wall,  whose 
plane  of  rupture  determines  the  direction  of  the  pressure. 
The  true  centre  of  gravity  of  the  sliding  wedge  then  is  the 
centre  of  gravity  of  this  reduced  polygonal  figure,  and  the 
point  at  which  a  line  drawn  through  it  parallel  to  the  plane 
of  rupture  interprets  the  perpendicular  at  the  back  of  the 
wall  is  the  point  of  application  of  the  horizontal  thrust. 


150  DREDGING  ENGINEERING 

Sluiceways. — The  structure  through  which  the  pumped 
water  escapes  from  the  impounding  basin  is  called  variously 
the  sluiceway,  sluicebox,  sluice  or  waste-weir.  The  prin- 
cipal parts  of  a  complete  sluice  are,  first,  the  box  proper, 
consisting  of  floor  and  sides  to  retain  the  dike  material; 
second,  one  or  more  sheet  pile  cut-off  walls  to  prevent  the 
percolation  of  water  (and  resultant  scour)  through  the  dike 
under  and  alongside  the  sluice-box  proper;  third,  the  weir 
itself,  built  up  of  a  series  of  planks,  set  loosely  in  vertical 
grooves  so  that  the  elevation  of  the  crest  of  the  weir  may 
be  raised  from  time  to  time  by  placing  additional  weir 
boards  as  the  filling  progresses,  or  lowered  to  drain  the 
basin;  fourth,  the  tide  gates,  which  are,  in  effect,  large 
flap  valves,  opening  in  the  direction  of  the  effluent  and  pre- 
venting the  back  flow  into  the  basin  of  outside  water  under 
heads  created  by  tides,  freshets,  or  other  causes;  fifth, 
a  walkway  across  the  sluice  for  the  convenience  of  the 
bank  patrol  and  to  facilitate  the  addition  or  removal  of  weir 
boards.  While  the  details  of  design  vary  materially  with 
the  various  types  of  dike  in  which  the  sluice  is  built,  the 
above  features  must  all  be  taken  care  of.  Fig.  49,  page 
151,  shows  the  type  of  sluice  used  for  earth  embankments 
in  connection  with  the  dredging  appurtenant  to  the  con- 
struction of  the  Hog  Island  Shipyard. 

The  argument  used  for  the  determination  of  pressure 
against  the  dike  is  applicable  also  to  the  sluice.  In  very 
soft  material,  it  may  be  necessary  to  found  the  structure 
upon  piles.  Revetment  work  of  sand  bags  or  rip  rap  upon 
the  adjacent  slopes  may  be  required  in  some  instances. 
A  sluiceway  built  through  a  crib  or  mud-fence  dike  may 
require  an  outboard  apron  to  prevent  back  scour. 

The  elevation  of  the  floor  of  the  sluice  will  be  such  that 
the  basin  can  be  drained  to  the  minimum  desired  level. 
The  requisite  width  of  the  box,  or  length  of  the  weir,  de- 
pends upon  the  number  and  capacity  of  the  dredges  dis- 
charging into  the  basin.  It  is  desirable  that  the  head  of 
water  on  the  weir  crest  be  not  greater  than  about  6  to  8 
inches,  in  order  that  the  disturbing  influences  of  high 


PRELIMINARY  CONSTRUCTION 


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152  DREDGING  ENGINEERING 

velocity  and  scour  be  reduced  to  a  minimum.     Thus,  the 
customary  width  of  sluices  is  about  as  follows: 

12  to  16  feet  for  one     20  inch  dredge  or  equivalent 
30  feet  for  two    20  inch  dredges  or  equivalent 
40  feet  for  three  20  inch  dredges  or  equivalent 

Let  us  investigate  a  sluice  designed  to  take  the  output 
of  one  20  inch  machine  with  an  average  discharge  through 
the  length  of  line  used  of  22.0  cu.  ft.  per  second.  Francis' 
weir  formula 

Q  =  3.33  bH* 
gives  results  sufficiently  close  for  our  purpose,  where 

Q  =  the  discharge  in  second  feet  =  22.0 
b    =  length  of  crest  =  width  of  sluice 
H  =  head  above  the  crest  measured  some  distance' 
back  from  the  crest. 

Then,  for  the  maximum  condition  of  H  =  8  in. 

fc  Q 

~  3.33  X  H* 

_  22.0 

=  3.33  X  (0.67)* 

from  which,     b  =  12  ft.  (approx.) 

Pipe  Lines. — The  shore  pipe  is  most  conveniently  laid 
upon  the  surface  of  the  ground,  but  the  necessity  often 
arises  for  the  depression  of  the  pipe  under  roadway  and 
track  crossings  and  for  the  elevation  of  the  line  upon  tres- 
tles in  crossing  swamp  land,  or  submerged  flats  on  which  the 
water  is  too  shallow  to  float  pontoons.  Such  depression 
generally  requires  no  special  structure,  as  the  usual  pipe 
will  stand  up  under  the  load  if  at  least  9  inches  below  the 
base  of  rail  and  if  the  two  adjacent  ties  are  spread  to  dis- 
tribute the  load  to  either  side.  Pipe  trestles  over  water 
are  generally  built  by  driving  two-pile  bents  about  12  feet 
on  centres,  with  the  piles  from  6  to.  8  feet  apart  in  the  bents, 
and  clamping  the  two  with  one  or  two  horizontal  members 
of  3  X  12  or  4  X  12,  which  support  the  pipe.  As  a  rule 


PRELIMINARY  CONSTRUCTION 


153 


no  transverse  or  longitudinal  bracing  is  necessary  except 
in  the  last  3  to  6  bents  and  in  the  ball  joint  platform,  where 
the  pull  exerted  by  the  pontoon  line  must  be  taken  care  of. 
For  the  same  reason,  a  short  stretch  of  the  shore  line  at  the 
point  of  connection  of  shore  and  pontoon  lines  is  tied  to- 
gether with  chain  or  cable. 


FIG.  50. — Pipe  lines  on  pile  trestle. 

For  maximum  efficiency,  the  land  pipe  line  should  be 
laid  out  with  minimum  curvature,  horizontally  and  verti- 
cally, and,  especially,  should  abrupt  changes  in  direction 
be  avoided. 

The  location  and  length  of  pipe  trestles  must  generally 
be  such  as  to  permit  the  dredge  to  reach  all  parts  ofjthe 
zone  ascribed  to  her  without  an  impracticable  length  or 
curvature  of  the  pontoon  lines.  Machines,  unless  built 
for  a  special  purpose,  are  seldom  equipped  with  more  than 
1200  feet  of  pontoon  line. 


CHAPTER  XI 
OPERATING 

Organization. — The  usual  method  of  performing  a  dredg- 
ing project  involves  a  dual  organization,  the  dredging 
contractor's  and  that  of  the  individual,  corporation  or 
governmental  department  for  whom  the  work  is  being  done. 
The  exceptions  are  the  operation  of  dredges  by  governments 
and  by  owners  upon  their  own  work.  We  will  not  attempt 
a  discussion  of  the  organization  requirements  of  the  con- 
tractor. They  will  depend  upon  the  size  of  his  plant,  its 
distribution  and  the  extent  of  his  yard  facilities  for  plant 
construction  and  repair. 

The  organization  necessary  to  handle  the  job  for  the 
party  having  the  work  done  consists  of  inspectors,  sta- 
tioned upon  the  dredges,  a  superintendent  and  a  small 
field  office  force  to  take  care  of  accounts,  records  and 
statistics.  On  machines  working  24  hours  per  day,  it  will 
generally  be  necessary  to  place  two  inspectors,  one  taking 
the  day  and  the  other  the  night  shift.  If  the  payment 
for  dredgings  is  based  upon  place  measurement,  the  full 
or  partial  services  of  a  hydrographer's  force  will  be  required, 
comprising  one  or  more  survey  parties,  a  small  office  for 
recording  and  plotting  and  an  engineer  in  charge.  Their 
duties  will  include  all  surveys  and  all  the  field  engineering 
appurtenant  to  the  dredging.  Direct  pumping  into  im- 
pounding basins  may  require  24  hours  of  bank  patrol  in 
two  or  three  shifts  of  so-called  dike-men,  in  numbers 
sufficient  to  take  care  of  ordinary  dike  maintenance.  If 
the  contract  is  such  that  the  contractor's  responsibility 
terminates  with  his  pontoon  line,  a  gang  of  men  is  needed 
to  handle  the  shore  pipe  lines.  Whatever  the  magnitude 
of  the  operation  justifies,  let  it  be  borne  in  mind  that  the 
key  to  successful  and  efficient  organization  is  clear  defini- 
tion of  duty  and  responsibility. 

154       • 


OPERATING 


155 


Cut  and  Range  Layout. — In  order  that  dredges  may  work 
to  the  best  advantage  and  make  a  uniform  bottom,  it  is 
customary,  where  practicable,  to  divide  the  area  to  be 
dredged  into  a  series  of  parallel  bands  or  belts,  called 
"cuts,"  denned  by  ranges  marked  by  flags  or  targets  set 
up  on  shore  or  on  stakes  or  piles  in  the  water.  Buoys  are 
sometimes  used  for  range  markings,  but  are  objectionable 
in  their  variation  of  position  within  the  radius  of  their 
mooring  cables.  The  dredge  operator,  by  lining  himself 


Cots 

A"   Cuts   for  Buckets  Dredges 
"2J"      "        "  Hydraulic 


FIG.  51. — Cut  and  range  layout. 

in  with  the  targets,  maintains  his  position  in  the  cut  be- 
tween arpair  of  ranges.  The  cuts  are  designated  by  some 
convenient  notation,  numbers  or  letters  or  a  combination 
of  the  two.  A  record  of  the  area  dredged  can  then  readily 
be  kept  by  having  the  inspector  report  the  cut  in  which  he 
is  working  and  the  distance  advanced  in  that  cut  daily. 
A  plan  of  the  cut  layout  is  therefore  very  useful  both  to 
field  and  office  forces.  The  information  shown  on  it 
should  embody  not  merely  the  lines  and  widths  of  the  cuts, 
but  also  the  range  targets,  the  specified  cross  section  and  the 
relative  position  of  the  cuts  thereto.  Such  a  plan  is  shown 
in  Fig.  51,  page  155.  It  is  not  necessary  for  grapples  and 
dippers  to  have  a  range  for  each  side  of  each  cut.  Every 


156  DREDGING  ENGINEERING 

other  cut  is  enough,  as  the  runner  can  line  in  with  his  port 
side  on  one  and  his  starboard  on  the  other. 

There  are  several  practical  considerations  that  are 
pertinent  factors  in  drawing  up  a  workabe  cut  layout. 
In  the  first  place,  the  width  of  cut  adopted  must  be  suitable 
to  the  machine  used.  Grapple  dredges,  loading  scows, 
can  make  a  cut  from  10  to  20  feet  wider  than  their  beam, 
because,  while  they  are  limited  on  one  side  by  the  scow, 
they  can  swing  on  the  other  somewhat  beyond  the  line  of 
the  hull  produced.  Thus  a  dredge  of  40  ft.  beam,  swinging 
a  bucket  of  from  6  to  10  yards  capacity  on  a  boom  of  the 
usual  length  for  loading  scows,  can  dredge  a  width  of  about 
50  to  60  feet  without  lateral  movement.  The  same  is  true 
of  dipper  machines  in  shoal  digging,  but  when  dredging 
to  more  than  medium  depths,  the  side  reach  of  the  dipper 
is  decreased,  due  to  the  increased  length  of  dipper  stick 
required  to  reach  the  bottom,  so  that  the  width  of  cut  may 
be  limited  even  to  the  width  of  the  hull.  Hydraulic  dredges 
of  the  swinging-ladder  type  will  usually  cut  a  swath 
approximately  equal  to  or  perhaps  slightly  greater  than 
the  width  of  their  hulls,  i.e.  about  40  to  50  feet.  Hydraulic 
machines  that  swing  about  a  stern  spud  must  work  in  a  cut 
about  150  to  200  feet  wide,  for  the  reason  that  the  dis- 
tance which  they  step  ahead  by  alternating  the  spuds  is 
determined  by  the  length  of  the  arc  traversed.  In  light 
digging,  therefore  (i.e.  where  the  depth  of  cut  is  small) 
the  minimum  width  of  cut  is  greater  than  in  heavy  cutting, 
because  the  distance  advanced  at  each  step  must  be  greater 
in  the  former  to  maintain  the  pump  feed  at  its  maximum 
capacity. 

Secondly,  it  is  preferable  that  the  direction  of  the  cuts 
be  parallel  to  the  current.  Dredging  at  an  angle  with  the 
direction  of  flow,  or  "cross  tide/'  involves  inconvenience 
and  at  times  real  difficulty  in  overcoming  the  resistance  of 
the  current  to  holding  and  advancing  the  dredge  in  the  cut. 

When  the  original  depth  of  water  is  too  little  to  float 
scows,  it  becomes  necessary,  if  the  expense  of  pilot  machines 
is  to  be  avoided,  to  dredge  always  on  the  edge  of  the  bank 


OPERATING  157 

and  always  in  that  direction  which  places  the  scow  on  the 
deep  water  side  of  the  dredge,  and  the  cut  layout  must  be 
planned  accordingly. 

The  marginal  cut  must  be  so  placed  relative  to  the  side 
slope  that  the  resulting  bottom  width  of  channel  from  toe  to 
toe  of  banks  shall  be  as  specified.  In  other  words,  the  cut 
will  extend  part  way  up  on  the  slope  so  that,  after  it  has 
been  dredged  to  the  full  depth  and  width,  the  sloughing 
in  of  the  bank  to  its  natural  angle  of  repose  will  place  the 
toe  of  the  slope  in  the  location  specified. 

Finally,  the  locations  of  pipe  trestles  and  the  cuts  for 
hydraulic  machines  shall  be  coordinated  to  give  pontoon 
lines  of  workable  and  economic  curvature  and  length. 

Hydrography. — The  field  engineering  appurtenant  to  the 
operation  of  a  dredging  project  comprises  the  following: 

1.  The  cut  layout  having  been  planned,  it  is  the  duty  of 
the  hydrographer  to  compute  the  position  of  the  range 
targets  with  respect  to  known  stations,  to  locate  them  in 
the  field,  set  them  up,   and  maintain  them.     To  avoid 
possible  confusion  where  the  number  of  ranges  is  large, 
it  is  desirable  to  employ  distinguishing  symbols  for  the 
targets.     A  convenient  method  is  the  use  of  timber  frames 
built  in  various  forms,  such  as  shown  in  Fig.  49,  elevated 
upon  poles.     Flags  of  different  colors  are  sometimes  used. 

2.  Tide  gauges  must  be  set  and  maintained  at  points 
from  which  they  can  be  read  by  the  dredge  operators. 
They  are  simply  vertical  planks  graduated  in  feet  with  the 
zero  at  the  datum  elevation. 

3.  If  the  material  is  removed  hydraulically,  involving 
periodical  estimates  of  the  amount  dredged  by  measure- 
ment in  place  in  the  cut,  soundings  are  taken  over  the  area 
before  and  after  dredging,  and  plotted  in  the  office  upon 
one  tracing,  distinguishing  between  the  two  surveys  by 
underlining  each  individual  sounding  of  one  set,  or  by  some 
similar  notation.     The  volume  is  then  computed  as  de- 
scribed under  "  Preliminary  Engineering/' 

4.  Shoals,  that  exist  after  dredging,  are  located  by  sound- 
ing or  " sweeping"  and  charted  for  the  information  of  the 


158  DREDGING  ENGINEERING 

Captain  and  inspector  of  the  dredge  assigned  to  their 
removal,  which  is  then  termed  a  "lumping"  machine. 
"Sweeping"  may  be  defined  as  the  operation  of  passing 
over  the  dredged  area  a  submerged  straight  edge,  suspended 
horizontally  at  the  specified  depth,  to  detect  the  presence 
of  shoals  or  lumps  protruding  above  that  depth.  Steel 
rails  or  structural  shapes  are  used  for  the  purpose,  hung  in 
such  a  way  that  the  men  on  the  float  above  are  immediately 
aware  of  any  interruption  to  smooth  progress.  With  a 
sweep  as  long  as  60  feet,  and  the  float  propelled  by  a 
motor  boat,  a  considerable  area  can  be  covered  in  a  short 
time.  The  obstructions  so  found  are  investigated  by  lead 
line  or  diver  or  both  as  the  necessity  requires  and  are  located 
by  instrument  or  range  intersection  and  charted  for  removal. 

5.  The  quantity  of  overdepth  dredging  in  excess  of  the 
allowance  is  computed  from  after  dredging  soundings.     The 
survey  establishing  the  shoals  may  serve  this  purpose  also. 

6.  Where  a  great  many  dredges  are  working  in  a  confined 
space  in  close  proximity  to  one  another,  the  difficulty  of 
isolating  the  area  covered  by  each  machine  becomes  acute, 
more  especially  in  the  case  of  hydraulic  machines  working 
for  place  measurement  compensation.     Under  these  condi- 
tions, it  is  helpful  to  plot  the  dredge  locations  at  intervals 
of  one  or  two  days,  as  found  by  angular  intersection  with 
sextant  or  transit,  dating  the  successive  positions  on  the 
map.     In  locating  swinging  hydraulic  machines,  which  are 
constantly  changing  position,  it  is  enough  simply  to  locate 
the  pivotal  spud,  which  can  then  be  plotted  on  the  chart 
as  a  point  or  small  circle,  with  an  arrow  pointing  from  it 
in  the  direction  of  the  advance,  and  drawn  to  scale  equal  in 
length  to  the  dredge  from  spud  to  cutter  head. 

7.  Contracts  for  hydraulic  plant  sometimes  provide  that 
payment  is  in  some  way  dependent  upon  the  length  of  pipe 
line  in  use,  in  which  case  it  is  advisable  that  the  inspectors7 
reported  lengths  be  checked  occasionally  by  actual  tape- 
line  measurement. 

Inspection. — The  inspector  is  the  dredge  time  keeper  and 
material  clerk.     Upon  him,  the  office  depends  for  all  in- 


OPERATING 


159 


formation  necessary  for 
complete  statistics  and 
determination  of  amount 
of  compensation. 

Let  us  first  consider 
the  duties  of  an  in- 
spector on  a  grapple  or 
dipper  dredge.  To 
facilitate  and  standard- 
ize the  data  furnished 
by  him,  he  is  provided 
with  three  sets  of  blank 
forms  and  a  log  book. 
One  form  is  the  bill  of 
lading,  bound  in  books 
of  about  100  each,  so 
that  the  stubs  may  be 
retained  therein  for 
record  in  their  proper 
sequence.  A  bill  of 
lading  is  made  out  by 
the  inspector  for  each 
scow  of  material  shipped 
by  the  dredge  and  num- 
bered in  chronological 
order.  A  typical  bill  is 
shown  in  Fig.  52.  On 
the  reverse  side  of  each 
bill  is  a  diagram  of  the 
plan  of  the  scow,  upon 
which  the  inspector 
designates  the  loading 
condition  of  each  pocket, 
whether  "F"  for  full, 
"E"  for  empty,  or  the 
amount  of  cut  for  pockets 
partially  loaded.  The 
inspector  is  furnished 


g    E  -o 
|  »l 

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160  DREDGING  ENGINEERING 

with  data  as  to  the  capacity  of  each  scow,  of  each  pocket 
and  the  deduction  for  one-tenth  foot  of  coaming  for  each 
pocket,  from  which  he  is  able  to  estimate  the  amount 
shipped,  whether  a  full  or  partial  load. 

The  bill  of  lading  goes  with  the  scow  to  the  place  of  dis- 
charge, whether  free  dump  or  rehandling  basin.  If  the 
former,  the  inspector  on  the  tug  fills  in  the  amount  dumped 
and  place  of  dumping,  deducting  any  material  that  is  lost 
in  transit.  He  then  signs  and  returns  the  bill  to  his 
employer.  If  the  material  is  to  be  pumped  ashore,  the  in- 
spector on  the  rehandling  machine  performs  the  same  func- 
tion since,  then,  there  is  no  necessity  for  an  inspector  on  the 
tow.  At  the  end  of  each  month  the  bills  and  stubs  afford 
a  complete  record  of  the  yardage  shipped  and  where 
dumped. 

The  second  printed  form  is  the  daily  report,  containing, 
in  addition  to  the  numbers  and  contents  of  the  scows 
loaded,  a  detailed  log  of  the  events  of  the  day  as  well  as  a 
summary  of  the  depth  information,  all  as  shown  on  the 
specimen  blank,  Fig.  53,  page  161. 

The  third  sheet  is  intended  as  a  convenience  to  the 
inspector  in  recording  and  tabulating  the  results  of  his 
before  and  after  dredging  soundings. 

The  log  book  is  simply  a  duplicate  record  of  the  informa- 
tion submitted  on  the  daily  report. 

The  contractor,  too,  keeps  his  records,  so  that  there  is 
an  abundance  of  checking  data  available,  leaving  little 
opportunity  for  paucity  of  evidence  in  the  event  of  dispute. 

The  stationary  hydraulic  dredge  inspector  has  a  some- 
what different  class  of  work.  He  has  no  bills  of  lading  since 
there  are  no  scows.  His  sounding  sheet  is  the  same,  but 
the  daily  report  is  quite  different — Fig.  54,  page  162.  A 
study  of  it  clearly  defines  his  duties.  His  yardage  reports 
are  merely  approximations,  because  they  are  computed 
from  his  own  soundings  without  instrument  location,  the 
pay  quantities  being  based  on  engineers'  surveys.  The 
nature  of  the  material  he  ascertains  by  periodical  visits 
to  the  impounding  basin. 


OPERATING 


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162  DREDGING  ENGINEERING 


JLI.S.C.  68-C-U65 

American  International  Shipbuilding  Corporation 

Daily  Report  of  Hydraulic  Dredge 

Contract  with dated 191 

Dredging  at .on —191 

Cat  No Station  So. JWidth_o.f_c.ut feet 

Total  advance  in  cut  to-day feet.      Depth  of  cut. £eet 

Approximate_a.nn.fi.u_n_t  dredged cubic  yards. 

REMARKS 

(  Describe  kind  of  material,  where  deposited  and  note  aty  iuformttion  necessary  to  a  clear  understanding  of  the  work.) 


Dredge  Captain  Inspector 

FIG.  54a. — Daily  report — hydraulic  dredge. 


OPERATING  163 

Distribution  of  Effective  Working  Time  Distribution  of  Time  Lost 

Pumping Hrs Min.  Repairs  to  Dredge Hrs .Min. 

Handling  Pipe  Line Hrs Min.  Repairs  to  Floating  Pipe  Line Hrs .Min. 

Handling  Lines  and  Anchors Hrs Min.  B.epairs  to  Banks  or  Sluice Hrs.          Min. 

Moving  Dredge Hrs Min.  Weather Hr.s. .Min. 

flrs Min.  Coaling Hr.s.~_  _. Min. 

Hrs. Min.  _. Hr_s____Min. 

Hrs_ Min.  Hrs JMin. 

JIrs Min.  .Hr.s Min. 

Hrs Min.  Hrs. Mm* 

_ Hrs Min.  Hrs. Mini 

_ Hrs. Min.  Hrs.          Min, 

JELrs. Min.  Hr.s.____Min. 

Total  Turi.e  Worked Hre Min.  Total  Time  Lost Hrs M.in. 

NOTE:  24  hours  per^day  must  be  accounted  for. 

PIPE   LINES 

Floating  Pipe  Line  Land  Pipe  Line 

Feet  In  Use  12.01  A.M.  Feet  In  Use  12.01  A.M. 


MISCELLANEOUS   INFORMATION 

Weather Coal  Used Tons 


(  Space  belo\v   this  line  not  to  be  filled  in    by  inspector  ) 


Time  Worked Hx>u.tB _Min> 

Time  Lost  Chargeable  to  A.I.S.C.at  Full  Bate Hours Min. 

Time  Lost  Chargeable  to  A.  I.  8.  Cat  Hall  Bate Hours Min. 

Time  to  be  Paid  For .Hours Min. 

Time  Lost  Chargeable  to  Contractor . Ho.urs Mi.n. 

lotal    Time  Accounted  For HQUJS Min. 

Average  Length  of  Pipe  Line  In  Use  During  Day Feet 

Fia.  546.— Reverse  side  of  report 'of  Fia.  54a. 


164  DREDGING  ENGINEERING 

The  inspector's  further  duties,  whether  he  be  a  bucket 
or  hydraulic  man,  comprise  reporting  promptly  to  the  office 
the  absence  of  range  targets  and  tide  gauges  and  any  devia- 
tions from  the  specifications  or  contract. 

It  is  apparent  that  the  reported  information  must  be 
particularly  accurate  and  in  greater  detail  for  machines 
working  under  contracts  of  lease,  as  the  time  element  then 
determines  the  amount  of  compensation. 


FIG.  55. — Spoil  bank — 20  inch  dredge.      (Courtesy  of  Morris  Machine  Works.) 

Basin  Regulation. — A  hydraulic  fill  builds  up  most 
rapidly  near  the  mouth  of  the  discharge  pipe,  sloping 
away  toward  the  dikes  and  sluice.  It  is  impossible  to  make 
a  fill  in  such  a  way  that  the  finished  surface  will  be  a  level 
plane,  but  is  is  practicable  to  approximate  this  condition 
if  desired  by  frequent  changes  in  the  location  of  the  point 
of  discharge  through  the  extension  of  the  line  by  adding 
pipe  sections  or  through  shifting  the  line  laterally.  The 
use  of  a  Y  and  gate  valves  on  the  fill  facilitates  both  the 
uniform  distribution  of  the  deposit  and  the  addition  of 
pipe  without  causing  the  dredge  to  stop  pumping.  As  the 
fill  rises,  it  becomes  necessary  to  increase  the  elevation 


OPERATING  165 

of  the  crest  of  the  sluice  by  placing  additional  weir  boards, 
in  order  that  the  quantity  of  water  retained  within  the  basin 
—or  in  other  words  that  the  impounding  area — shall  be 
great  enough  to  allow  such  quantity  of  sedimentation  there- 
in as  to  preclude  the  loss  of  a  large  percentage  of  solid 
matter  by  being  carried  away  in  suspension  in  the  effluent. 
It*  is  not  that  such  loss  is  disadvantageous  to  the  party 
making  the  fill  (rather  is  it  a  benefit  as  an  inexpensive 
method  of  disposal  of  dredgings),  but  that  a  rich  weir 
discharge  will  result  in  shoaling  outside  of  the  sluice  or  in 
adjacent  waterways  and  channels,  to  the  detriment  of 
adjoining  property  owners  and  the  maintenance  of  nearby 
channels.  For  this  reason,  the  War  Department  has  abso- 
lute authority  to  restrict  the  amount  of  solid  effluent. 
Samples  of  the  sluice  water  are  taken  in  jars  from  time  to 
time  and  allowed  to  stand  until  they  clarify  and  the  per- 
centage of  solids  noted.  The  addition  of  a  little  alum  will 
accelerate  the  precipitation.  The  allowable  percentage 
depends  upon  the  conditions.  The  sluice  may  be  so  located 
as  to  cause  little  or  no  disadvantage  from  outflowing  ma- 
terial or,  on  the  other  hand,  its  location  relative  to  dredged 
channel  may  result  in  serious  shoaling  therein.  Should  it 
become  necessary  to  continue  pumping  into  the  basin 
after  the  sluice  weir  has  attained  its  full  height  and  the 
effluent  has  become  too  rich  for  acceptance,  the  impounding 
area  may  be  increased,  in  effect,  by  the  interposition  of 
baffles  between  the  pipe  discharge  and  the  sluice,  causing 
the  water  to  pursue  a  more  devious  course  to  the  weir 
and  thereby  increasing  the  opportunity  for  sedimentation. 

It  is  often  helpful  to  drain  the  basin  occasionally  when 
approaching  repletion,  allowing  the  mud  to  compact  or 
set,  thus  rendering  the  effluent  more  lean  and  relieving  to 
some  extent  the  pressure  on  the  dikes. 

Dike  maintenance  is,  of  course,  a  feature  of  " operating.'7 
The  destructive  agencies  to  be  guarded  against  and  the 
methods  of  protection  have  been  outlined  under  the  heading 
"  Dikes  for  Impounding  Basins." 

The   quantity  of  pumpings  entering  a  basin  may  be 


166 


DREDGING  ENGINEERING 


approximated  by  measurements  of  the  rectangular  coordi- 
nates of  the  curve  of  the  pipe  discharge  with  respect  to  the 
end  of  the  pipe.  With  the  pipe  running  full  and  the  end 
section  horizontal,  measure  the  vertical  and  horizontal 


15 


10 


5  10 

Velocity  in  Ft.  per  Sec. 

o 


15 


X  =  Vt 

y  =  l/2gt2 

Eliminating  t 

v  =~2^~y  * 

V2  =  16.08  (y) 
and     Q  =  av 
FIG.  56. — Curve  for  determining  discharge  from  the  coordinates. 

distance  from  any  point  on  the  centre  of  the  curve  to  the 
centre  of  the  end  of  the  pipe.  A  photograph  will  facilitate 
the  measurement,  first  fixing  the  point  to  be  measured  by 
placing  a  batten  horizontally  in  the  plane  of  and  a  short 
distance  below  the  pipe.  Referring  to  Fig.  56,  page  166, 


OPERATING 


167 


let  X  and  Y  be  the  coordinates  of  the  point  P  referred  to 
the  origin  0. 

If  y  =  velocity  in  feet  per  sec. 
T  =  time  in  sees. 

and  G  =  acceleration  of  gravity  in  feet  per  sec.  =  32.16 

then      X  =  VT 

and     F  =       GT* 


eliminating  T,  V2  =  ~  ( -^  )  = 


FIG.  57. — A  powerful  discharge;  velocity  about  18  ft.  per'second. 
Morris  Machine  Works.) 


(Courtesy  of 


Solve  for  V,  after  which  the  quantity  Q  in  cu.  ft.  per  sec. 
follows  from  the  formula  Q  =  AV  here  A  =  the  sectional 
area  of  the  pipe  in  sq.  ft.  The  curve  of  Fig.  56,  page  166, 
plotted  from  the  above  formula,  gives  the  velocity  directly 

/X2\ 
for  any  value  of  the  ratio  f  y~j. 

Progress  Keeping. — Complete  progress  information  em- 
bodies records  of  three  distinct  elements,  yardage  removed, 
area  covered  and  time  distribution,  each  of  which  is  col- 
lected, compiled  and  presented  in  some  concise  form  from 


168  DREDGING  ENGINEERING 

which  the  important  facts  may  quickly  be  assimilated. 
The  graphical  method  is  easily  the  most  legible  and  may  be 
employed  in  all  three  cases. 

The  daily  output  of  each  dredge,  or,  if  desired,  the  out- 
put of  a  group  of  dredges  working  in  one  zone  as  scheduled, 
is  plotted  vertically  with  respect  to  working  days  on  the 
horizontal  axis,  and  the  resulting  curve  will  show  at  a  glance 
the  fluctuations  in  performance.  A  straight  horizontal 
line  representing  the  assumed  daily  average  yield  of  the 
dredge,  or  group  of  dredges,  is  drawn  to  the  same  axes. 
The  area  included  below  the  curve  must  be  at  least  equal 
to  that  below  the  straight  line  if  the  dredge  is  up  to  the 
schedule.  On  a  second  set  of  axes,  the  accumulated  yard- 
age is  plotted  with  reference  to  working  days,  showing  the 
total  yardage  to  date  at  any  time.  By  comparing  this 
curve  with  a  line  drawn  from  the  same  origin  on  the  same 
sheet  representing  the  total  scheduled  yardage  to  date  of 
any  day  (which  line  is  called  the  " bogie'7)  the  number  of 
days  or  cubic  yards  by  which  the  dredge  is 'behind  schedule 
is  obvious.  The  vertical  axis  of  the  same  sheet  may  also 
be  graduated  to  give  the  percentage  of  completion  at  any 
time.  Additional  "bogies"  and  curves  may  be  drawn  for 
the  summarized  output  of  all  the  dredges  on  the  job,  or 
of  all  the  bucket,  or  all  the  hydraulic  machines.  Such 
graphs  are  very  helpful  in  adhering  to  a  dredging  schedule. 

The  record  of  area  covered  is  best  kept  on  a  map  showing 
the  cut  layout,  by  coloring  the  area  dredged  each  day  by 
each  machine.  Different  colors  are  used  for  different 
depths,  and  the  distinction  between  areas  reported  dredged 
and  those  shown  by  final  survey  to  be  down  to  depth  may 
conveniently  be  indicated  by  cross  hatching  the  former 
and  solidly  coloring  the  latter. 

The  graphical  distribution  of  time  is  of  value  in  bringing 
out  for  each  dredge  the  percentages  of  lost  time  due  to 
various  causes,  such  as  lack  of  plant  (waiting  for  scows  or 
tug-boats),  lack  of  supplies  (coal,  etc.),  repairs,  weather, 
tide,  etc.  One  method  divides  a  rectangle  into  areas  of 
different  colors,  each  indicative  of  one  class  of  lost  time  or 


OPERATING  169 

of  working  time  and  representing  by  its  relative  length  the 
percentage  of  the  total  time  for  the  day,  or  week. 

This  question  of  distribution  of  time  and  the  percentage 
of  lost  time  due  to  various  causes  is  obviously  of  vital 
interest  to  the  contractor.  Dredging  operations  are  char- 
acterized by  few  units  of  large  capacity.  When  one  ma- 
chine is  idle,  a  big  percentage  of  the  work  is  at  a  standstill 
and  a  large  investment  becomes  unproductive.  Thus  it 
is  of  paramount  importance  that  all  the  units  be  kept  busy 
all  of  the  time  and,  to  this  end,  a  thorough  study  of  the 
lost  time,  its  cause  and  remedy  is  imperative. 


CHAPTER  XII 
REMOVAL  OF  SUB-AQUEOUS  ROCK 

As  has  been  said,  the  high-powered  ladder  dredges 
are  able  to  remove  by  direct  dredging  the  softer  rock  for- 
mations. When,  however,  the  rock  is  harder  than  lies 
within  the  capabilities  of  these  machines,  some  means  must 
be  used  to  prepare  the  rock  for  dredging,  comprising  the 
reduction  of  the  strata  to  relatively  smaller  pieces.  There 
are  two  general  methods :  first,  by  the  use  of  explosives  and, 
second,  by  the  use  of  rock-breaking  machines.  Again, 
there  are  two  general  methods  of  using  explosives :  first, 
by  undermining  and  blasting,  and,  second,  by  drilling  and 
blasting  from  the  surface.  This  last  is  by  far  the  more 
popular  method  in  this  country,  the  other  having  been 
used  to  a  limited  extent  only. 

The  problem  then  arises  as  to  whether  the  softer  rocks, 
such  as  corals,  soft  limestones  and  tufas,  are  more  econom- 
ically removed  by  large,  powerful  dredges  from  in  situ 
without  previous  blasting,  or  by  smaller,  less  expensive 
dredges  after  first  breaking  up  the  rock  by  blasting  or  by 
rock  breaking  machines.  On  the  one  hand,  we  have  the 
saving  effected  by  the  omission  of  drilling  and  blasting  to 
offset  the  increased  cost  and  rapid  depreciation  of  the  larger 
dredging  plant  over  a  longer  period  of  time  and,  on  the 
other,  the  saving  effected  by  the  use  of  lighter  dredging 
equipment  in  less  destructive  material,  over  a  shorter 
period  (because  of  increased  output)  to  offset  the  added 
cost  of  drilling  and  blasting.  The  question  apparently 
is  still  an  open  one  and  will,  of  course,  be  influenced  by  con- 
ditions peculiar  to  each  project. 

Undermining  and  Blasting. — The  method  of  rock  re- 
moval by  undermining  and  blasting  consists  in  the  sinking 

170 


REMOVAL  OF  SUB-AQUEOUS  HOCK  171 

of  vertical  shafts,  from  the  bottom  of  which  a  series  of 
horizontal  galleries  are  driven,  after  which  charges  are 
placed  in  drill  holes  in  the  remaining  supporting  and  over- 
lying rock  and  exploded  in  one  operation.  In  the  removal 
of  Flood  Rock,  New  York  Harbor,  General  John  Newton 
sank  two  shafts,  from  which  he  drove  a  series  of  galleries 
10  feet  square  at  right  angles  to  each  other,  leaving  a  roof 
supported  by  square  columns  of  rock  15  feet  on  a  side.  The 
roof  and  columns  were  drilled,  loaded  and  simultaneously 
fired  after  admitting  the  water.  The  total  cost  was  $5.88 
per  cu.  yd.,  of  which  $3.19  was  the  cost  of  the  subsequent 
dredging. 

Drilling  and  Blasting  from  the  Surface. — This  method 
comprises  the  drilling  of  a  series  of  vertical  holes  down 
into  the  rock  by  means  of  drills  mounted  on  floating  hulls, 
after  which  the  holes  are  loaded,  usually  from  the  deck 
by  means  of  a  charging  pipe,  and  then  fired.  The  holes 
are  drilled  at  the  corners  of  squares,  the  sides  of  which 
vary  with  the  character  of  the  rock  from  about  4  to  8 
feet.  In  order  that  the  rock  may  break  to  the  specified 
depth,  the  holes  are  drilled  to  several  feet  below  that  grade. 

From  the  original  humble  raft  carrying  one  drill,  the 
drill  boat  has  developed  into  the  modern  steel  hull  mounting 
as  many  as  five  or  more  traveling  drill  frames,  and  equipped 
with  spuds  or  columns,  upon  which  it  is  set  up  on  the  rock, 
free  from  wave  motion.  These  spuds,  one  at  each  corner 
of  the  hull,  are  being  forced  down  while  the  drills  are 
working  and  the  boat  is  raised  above  the  elevation  of 
normal  flotation,  in  which  position  it  is  maintained  by  the 
automatic  regulation  of  the  steam  pressure  in  each  spud 
engine.  The  drills  are  usually  of  the  steam-driven  per- 
cussion type  and  are  carried  on  traveling  steel  towers  from 
which  vertical  leads  extend  down  over  the  edge  of  the  hull, 
forming  guides  for  the  drill  cylinders.  The  drill  feed  and 
the  tower  travel  are  generally  power-operated. 

In  order  to  prevent  the  transmission  of  the  barge  motion 
to  the  drill  in  waters  of  wide  tidal  range,  the  drill,  guides 
and  feed  mechanism  are  mounted  upon  a  steel  column  in- 


172  DREDGING  ENGINEERING 

stead  of  the  tower.  The  base  of  the  column  rests  on  the 
rock  and  is  so  attached  as  to  permit  the  vertical  motion 
of  the  boat  independently  of  the  drill. 

The  essential  requirements  of  a  submarine  rock  drill  are 
great  striking  power,  quick  action,  strength  and  durability. 

Some  drill  barges  have  been  constructed  in  which  the 
drills  operate  through  a  central  well  in  the  hull.  Others  are 
equipped  with  pipes  for  enclosing  the  drill  in  passing  through 
the  overlying  soft  strata  to  keep  it  free  of  obstruction. 

Rock-Breaking  Machines. — In  this  method,  the  rock 
is  broken  up  by  the  impact  of  a  fulling  ram.  For  many 
years,  Lobnitz  &  Co.,  of  Renfrew,  Scotland,  have  manu- 
factured machines  of  this  kind.  The  ram  weighs  from  6 
to  15  tons  and  has  a  projectile  shaped  cutter-head.  They 
are  raised  from  6  to  15  feet  and  dropped  about  four  times 
per  minute.  The  concentrated  impact  is  capable  of  break- 
ing the  hardest  rock.  After  one  layer  is  broken  to  a  depth 
of  several  feet,  it  is  removed  by  dredging  and  a  second  layer 
attacked.  The  action  of  the  machine  is  partly  to  pulverize 
and  partly  break  the  rock. 

The  machinery  for  operating  the  rani,  comprising  an 
"A"  frame  and  hoisting  engine  with  friction  clutch,-  is 
mounted  on  a  barge,  held  in  position  by  cables  from  winches 
on  deck.  The  surface  of  the  rock  is  attacked  with  the 
breaker  at  intervals  of  about  four  feet  each  way.  The 
limit  of  penetration  (usually  about  3  feet)  is  obtained  at 
each  point  before  going  to  the  next. 

The  Submarine  Company,  of  New  York,  manufacture  a 
rock  breaker  in  which  a  heavy  hammer,  working  inside 
of  a  long  cylinder  suspended  from  the  "A"  frame,  strikes  a 
short  chisel  resting  on  the  rock  and  attached  to  the  lower 
end  of  the  cylinder  in  a  manner  permitting  a  limited  amount 
of  vertical  motion.  Greater  efficiency  is  claimed  because 
the  hammer  works  in  air,  the  water  being  excluded  by  com- 
pressed air. 

Breakers  of  the  Lobnitz  type  appear  to  be  the  most 
economical  means  of  preparing  hard  rock  for  dredging, 
when  the  rock  to  be  removed  is  less  than  two  feet  in  depth, 


REMOVAL  OF  SUB-AQUEOUS  ROCK  173 

but,  where  the  depth  of  cut  is  greater  than  two  feet,  drilling 
and  blasting  by  the  American  method  is  the  most  rapid 
and  economical.  In  rock  that  is  thinly  stratified  or  that 
shatters  readily,  the  limit  of  two  feet  for  the  economic  use 
of  the  Lobnitz  cutter  may  be  somewhat  increased. 

Dredging  the  Broken  Rock. — The  choice  of  a  dredge  for 
the  removal  of  the  rock,  broken  or  blasted  as  above,  lies 
between  the  dipper  and  the  ladder  types.  Except  where 
the  rock  is  broken  by  crushers  of  the  Lobnitz  type  to  a 
depth  of  only  two  or  three  feet,  the  dipper  dredge  is  to  be 
preferred.  Its  ability  to  handle  great  masses  by  virtue  of 
its  larger  bucket  effects  a  considerable  economy  through 
the  permission  of  wider  spacing  of  drill  holes.  The  cost  of 
the  drilling  and  blasting  is  usually  the  largest  part  of  the 
total  cost  of  the  rock  removal.  The  smaller  dimensions 
of  the  buckets  of  the  elevator  dredge  require  the  rock  to  be 
broken  into  smaller  fragments.  The  larger  pieces  are 
ejected,  requiring  subsequent  removal  by  a  grapple 
machine  or  other  means.  The  ability  of  the  ladder  type 
to  dig  uniformly  to  grade,  however,  renders  it  particularly 
adaptable  to  the  removal  of  rock  prepared  by  the  Lobnitz 
machine. 


INDEX 

PAGE 

"'A"  frame,  dipper  dredge 31-37 

grapple  dredge 16-19 

hydraulic  dredge 66-67 

"Admiral,"  The 23 

Agitation  method 93 

"Alfred  E.  Hunt,"  The 47 

"Alpha,"  The 87 

Angle  of  repose  of  materials 143 

Back-guys,  dipper  dredge 31-37 

grapple  dredge 16-19 

hydraulic  dredge 66-67 

Backing  chain 29 

"Baltic,"  The. 24 

Bank  (see  also  Dike). 

height  of 117 

patrol 154 

protection 127-128 

spoil .'; 116 

spuds 116 

Banking  machine 134 

Basin,  impounding 1,  113-114 

regulation 164-167 

rehandling 57,  115 

Bill  of  lading 159 

Boilers,  grapple  dredge 22 

hydraulic  dredge 82 

Boom,  dipper  dredge 29,  31-37 

grapple  dredge 16-19 

hydraulic  dredge 66-67 

log -. 22,  27 

swinging  by  bucket  wires 27,  29 

Booster  pump 83-86 

Borings. 99-100 

Bucket,  Arnold 13 

clam-shell 2,  6 

closing  power 10-11 

common  grab 6-11 

dipper  dredge 29,  31 

dorsey  wire 16 

grab  details . . .  : 15-16 

175 


176  INDEX 

PAOE 

Bucket,  hard-digging 6,  1 1 

ladder  dredge 50 

orange-peel 2,  6 

poles 13,  16 

side  boards 16 

single  wire 13 

sliding  cross-head 6,  12-13 

soft-digging 6,  1 1 

Stockton 13 

teeth 16 

Williams 13-14 

Bucket  dredges,  classification 2 

definition 1 

Bucyrus  dipper  dredge 29-30 

Bulkhead 132,  135 

relieving  platform 146-149 

Bull-wheel 29,  41 

"Camden,"  The 6 

Canalization 127 

"Caribbean,"  The 91 

"Cascadas,"  The 44-45 

Channel,  location , 98,  101 

maintenance 124,  127 

side  slopes 103 

"Charleston,"  The 90 

Clam-shell  dredge  (see  Grapple  dredge). 

Coaming 54 

"Columbia,"  The 24 

Condenser. ... , 23 

Contracts 118-121 

Costs,  estimating 123-124 

Crib  dikes 129,  132,  134-135 

Cross  sectioning . 101-102 

Crowding  engine 33,  35 

"Culebra,"  The. 91,  93 

Current,  drift - 98-99 

in  rivers 126-127 

transporting  power 126-127 

Cut,  advance  in 28,  59,  65-66 

depth  of 117 

layout 155-157 

pilot 116 

width  of 115,  156 

Cutter 59,  61-63 

engine 59,  82 

head 61 

shaft..  63 


INDEX  177 

PAGE 

"Delaware,"  The 93 

Depth  gauge 28 

Differential  drum 39 

Dike 101 

crib 129,  132,  134-135 

design 146-149 

dry 130 

earth 130-131 

permeable • 128 

pressures  on 139 

sheet-pile 128-129 

spur 127-128 

stone 128-130,  135 

training 127-128 

wet 132 

Dipper  dredge,  "A"  frame 31 

application 42 

back-guys 31 

bucket 29,  31 

Bucyrus 29-30 

boom ,  ...   29,  31-37 

definition 2 

description 29 

dipper  stick -. 29,  31-35 

high  powered , 44 

hull . 40 

machinery 39 

operation 41 

pin-up 37 

purchase-rigged 37 

spuds 37 

Dipper-stick 29,  31-35 

Displacement,  measurement  of  loads  by 106 

of  scows i 54 

Disposal  of  Dredgings 57,  107,  112,  115 

Dorsey  wire ,  .  .  .  .      16 

Dredge  wires 20-21,  28,  64-66 

Dredges,  adaptability  to  soils. 108-112 

classification 1-3 

depth  limitations 117 

Dredging,  contracts 118-121 

costs 113 

definition 1 

objects 95 

overdepth. , 103,  158 

plans  and  specifications 118-121 

Dredgings,  disposal  of 57,  107,  112,  115 

use  of 95 


178  INDEX 

PAGE' 

Drilling,  rock 171-172 

Dumps,  free 112 

Earth,  dikes 130-132 

pressure 143 

Elevator  dredge  (see  Ladder  dredge). 

Equivalent  fluid  pressure 146 

Estimating,  costs 123-124 

methods  of 101-103 

quantities 100-107 

Exploring  the  site 97-100 

Fill,  value  of , 114 

"Finn  MacCool,"  The 21,  24 

Flotation,  dredges 117 

pontoons 116-117 

scows . 116 

Flume 114 

Friction  head,  table 75-76 

Frictions 23 

Gallows-frame 4,  20,  22 

"Gamboa,"  The 44-45 

Gold  dredging .- -. 48-49 

Grapple  dredge,  "A"  frame. . .  ...    16-19 

back-guys 16-19 

boom 16-19 

bucket, 6-16 

classification 2 

definition 2,  4 

description 4 

gallows-frame 20,  22 

house 24 

hull....'. 24-26 

machinery 22 

operation 26 

spuds 20 

spud-wells 20 

Hogging 26,  55 

Hull,  dipper  dredge 40-41 

grapple  dredge 19,  24-26 

Hydraulic  dredge,  "A"  frame 66 

back  legs 66 

boom 66 

classification 3 

cutter-head 59,  61-63 

definition 1 


INDEX  179 

PAGE 

Hydraulic  dredge,  design 79-81 

feeding 64-66 

ladder 61,  63-64 

machinery 81 

Mississippi  River  type 86-88 

operation 83 

pipe  line 67-71,  152-153 

power 78-79 

pump 60,  71-81 

river  type 58-88 

radial  feeding 58-86 

sea-going 89-93 

solid  output 83 

spuds 64-66 

thrust  bearing '. 71,  82 

Hydrochronograph 97 

Hydrography .    157-158 

Ice  thrust 129 

Inspection 159-164 

Keelsons 26 

Ladder  dredge,  buckets 50 

classification 2 

definition 2 

description 50 

historical 46 

sea-going 52 

stationary 52 

Lobnitz  machine 172-173 

Log  book 160 

Lumping 157-158 

Machinery,  dipper  dredge 39 

grapple  dredge 22 

hydraulic  dredge 81 

Mechanics,  beam  with  varying  load 148 

Mud-fence 132-134 

design 146 

" Natomas  Consolidated,"  The 49 

"Newburgh,"  The 34 

"New  Jersey,"  The 82 

"Onondaga,"  The 44-45 

Organization 154 

Overdepth  dredging 103-104 


180  INDEX 

PAGE 

"Pacific,"  The ; 24 

"Paraiso,"  The 44-45 

"  Pennsylvania,"  The 59-60 

Pilot  cut. . 116 

Pipe,  ball-joint 70 

capacity  table 75-76 

discharge 67-68 

fittings 70 

hydraulic  dredge 68 

line 67-71,  152-153 

Place  measurements 157 

ratio  to  scow 103,  105-106 

Placer  dredge 48-49 

Plane  of  rupture 143 

Planimeter  method  of  estimating  quantities 101 

Plans,  dredging 1 18-121 

Plant,  choice  of 107-118 

Pontoon,  hydraulic  dredge 70-71 

line 67 

Position  control 4,  20-22 

"President,"  The 38,  40 

Pressure  on  dikes  and  bulkheads 146-149 

Prismoidal  formula 102 

Probings 99 

Progress  keeping 167-169 

Pump,  booster 83-86 

design , 79-81 

drive 81 

grapple  dredge 23 

hydraulic  dredge 60,  71-81 

Pumpings,  measurements  of 157,  165-167 

percentage  of  solids 83 

Rainfall 125 

Range,  layout 155-157 

Rehandling 57,  115 

Reports,  daily : 160-164 

Rivers,  currents , , 126 

sediment  transportation 126-1 27 

Rock,  removal 170-173 

dredging 42-44,  48,  173 

Run-off 125 

Sand  and  gravel  dredging 52 

Scheduling 121-122 

Scoop  dredge  (see  Dipper  dredge) 

Scour 101,  125-126 

earth  dikes..  131 


INDEX  181 

PAGE 

Scow,  bottom-dump 54-57 

concrete , 57 

deck 57 

handling 6,  27-28 

measure,  ratio  to  place 103,  105-106 

side-dump 57 

Sea-going  hydraulic  dredge 89-93 

Sediment,  transportation  of 126 

Sedimentation , 101,  126 

Shear  legs 29 

Sheaves,  bucket 8 

grapple  dredge ,    17-19 

spud .- 21 

Sheet  piles,  types 133 

design 146-148 

shear  and  moment 147-149 

Side  boards 16 

Sleeves 70 

Sliding  wedge 144-145 

Sluice  (see  Sluiceways) . 

Sluiceways 150-152 

effluent  from 114,  165 

Soundings 97-98 

Specifications 118-121 

Spud,  bank 116 

definition 4 

dipper  dredge 37 

grapple  dredge , 20-22 

hydraulic  dredge. 64-66,  87 

trailing 37,  39 

walking 37,  39,  59,  66 

walking  on 28 

wells 20-21 

Spur  piles 136-139 

Squatting  of  moving  vessels 106-107 

Strainers 62-63 

Strakes 26 

Suction  dredge  (see  Hydraulic  dredge). 

Suction  hopper  dredge  (see  Sea-going  hydraulic  dredge). 

Surveys 97-99,  113 

Sweeping 157-158 

"Tampa,"  The 60 

"Tellico,"  The .' .  .     40 

Thrust  bearing 71   32 

Tide-gauge 28,  97,  157 

"Toledo,"  The. ' 44 

Topping  fall 17?  18,  31,  35 

Towing m   113 


182  INDEX 

PAGE 
Unit  method  of  estimating  quantities .    101-103 

Waste-weir  (see  Sluiceways). 

Water  jet  agitator 87 

Wave  action. . . 129,  131 

Weight  of  materials 143 

Weir  boards,  use  of i 164-165 

Wires,  bucket. 7-10,  27,  29 

bucket  dredge 20-21,  28 

hydraulic  dredge 64-66 

scow  handling 27-28 


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